read.c: Fail if gainmap properties have more than one nclx

The current code simply adopts the first nclx property of the
gainmap item. The right behavior seems to be to fail if there are
multiple nclx properties (similar to what we do for the color image
and the tonemap image).

HEIF Section 6.5.5.1:
```
When two ColourInformationBoxes are associated with an image
item, one shall have a colour_type value of 'rICC' or 'prof'
(providing either restricted or unrestricted ICC profiles
respectively) and the other one shall have a colour_type value of
'nclx' with colour_primaries equal to 2 and
transfer_characteristics equal to 2 (2 indicating "unspecified",
since these data are supplied by the ICC profile instead).
```
diff --git a/src/read.c b/src/read.c
index ce09af4..1e6bd78 100644
--- a/src/read.c
+++ b/src/read.c
@@ -4673,6 +4673,48 @@
     return AVIF_RESULT_OK;
 }
 
+// On success, this function returns AVIF_RESULT_OK and does the following:
+// * If a nclx property was found in |properties|:
+//   - Set |*colorPrimaries|, |*transferCharacteristics|, |*matrixCoefficients|
+//     and |*yuvRange|.
+//   - If cicpSet is not NULL, set |*cicpSet| to AVIF_TRUE.
+// This function fails if more than one nclx property is found in |properties|.
+// The output parameters may be populated even in case of failure and must be
+// ignored.
+static avifResult avifReadColorNclxProperty(const avifPropertyArray * properties,
+                                            avifColorPrimaries * colorPrimaries,
+                                            avifTransferCharacteristics * transferCharacteristics,
+                                            avifMatrixCoefficients * matrixCoefficients,
+                                            avifRange * yuvRange,
+                                            avifBool * cicpSet)
+{
+    avifBool colrNCLXSeen = AVIF_FALSE;
+    for (uint32_t propertyIndex = 0; propertyIndex < properties->count; ++propertyIndex) {
+        avifProperty * prop = &properties->prop[propertyIndex];
+        if (!memcmp(prop->type, "colr", 4) && prop->u.colr.hasNCLX) {
+            if (colrNCLXSeen) {
+                return AVIF_RESULT_BMFF_PARSE_FAILED;
+            }
+            colrNCLXSeen = AVIF_TRUE;
+            if (cicpSet != NULL) {
+                *cicpSet = AVIF_TRUE;
+            }
+            *colorPrimaries = prop->u.colr.colorPrimaries;
+            *transferCharacteristics = prop->u.colr.transferCharacteristics;
+            *matrixCoefficients = prop->u.colr.matrixCoefficients;
+            *yuvRange = prop->u.colr.range;
+        }
+    }
+    return AVIF_RESULT_OK;
+}
+
+// On success, this function returns AVIF_RESULT_OK and does the following:
+// * If a colr property was found in |properties|:
+//   - Read the icc data into |icc| from |io|.
+//   - Sets the CICP values as documented in avifReadColorNclxProperty().
+// This function fails if more than one icc or nclx property is found in
+// |properties|. The output parameters may be populated even in case of failure
+// and must be ignored (and the |icc| object may need to be freed).
 static avifResult avifReadColorProperties(avifIO * io,
                                           const avifPropertyArray * properties,
                                           avifRWData * icc,
@@ -4685,37 +4727,19 @@
     // Find and adopt all colr boxes "at most one for a given value of colour type" (HEIF 6.5.5.1, from Amendment 3)
     // Accept one of each type, and bail out if more than one of a given type is provided.
     avifBool colrICCSeen = AVIF_FALSE;
-    avifBool colrNCLXSeen = AVIF_FALSE;
     for (uint32_t propertyIndex = 0; propertyIndex < properties->count; ++propertyIndex) {
         avifProperty * prop = &properties->prop[propertyIndex];
-
-        if (!memcmp(prop->type, "colr", 4)) {
-            if (prop->u.colr.hasICC) {
-                if (colrICCSeen) {
-                    return AVIF_RESULT_BMFF_PARSE_FAILED;
-                }
-                avifROData iccRead;
-                AVIF_CHECKRES(io->read(io, 0, prop->u.colr.iccOffset, prop->u.colr.iccSize, &iccRead));
-                colrICCSeen = AVIF_TRUE;
-                AVIF_CHECKRES(avifRWDataSet(icc, iccRead.data, iccRead.size));
+        if (!memcmp(prop->type, "colr", 4) && prop->u.colr.hasICC) {
+            if (colrICCSeen) {
+                return AVIF_RESULT_BMFF_PARSE_FAILED;
             }
-            if (prop->u.colr.hasNCLX) {
-                if (colrNCLXSeen) {
-                    return AVIF_RESULT_BMFF_PARSE_FAILED;
-                }
-                colrNCLXSeen = AVIF_TRUE;
-                if (cicpSet != NULL) {
-                    *cicpSet = AVIF_TRUE;
-                }
-                *colorPrimaries = prop->u.colr.colorPrimaries;
-                *transferCharacteristics = prop->u.colr.transferCharacteristics;
-                *matrixCoefficients = prop->u.colr.matrixCoefficients;
-                *yuvRange = prop->u.colr.range;
-            }
+            avifROData iccRead;
+            AVIF_CHECKRES(io->read(io, 0, prop->u.colr.iccOffset, prop->u.colr.iccSize, &iccRead));
+            colrICCSeen = AVIF_TRUE;
+            AVIF_CHECKRES(avifRWDataSet(icc, iccRead.data, iccRead.size));
         }
     }
-
-    return AVIF_RESULT_OK;
+    return avifReadColorNclxProperty(properties, colorPrimaries, transferCharacteristics, matrixCoefficients, yuvRange, cicpSet);
 }
 
 #if defined(AVIF_ENABLE_EXPERIMENTAL_GAIN_MAP)
@@ -4844,16 +4868,12 @@
         AVIF_CHECKERR(gainMap->image, AVIF_RESULT_OUT_OF_MEMORY);
 
         // Look for a colr nclx box. Other colr box types (e.g. ICC) are not supported.
-        for (uint32_t propertyIndex = 0; propertyIndex < gainMapItemTmp->properties.count; ++propertyIndex) {
-            avifProperty * prop = &gainMapItemTmp->properties.prop[propertyIndex];
-            if (!memcmp(prop->type, "colr", 4) && prop->u.colr.hasNCLX) {
-                gainMap->image->colorPrimaries = prop->u.colr.colorPrimaries;
-                gainMap->image->transferCharacteristics = prop->u.colr.transferCharacteristics;
-                gainMap->image->matrixCoefficients = prop->u.colr.matrixCoefficients;
-                gainMap->image->yuvRange = prop->u.colr.range;
-                break;
-            }
-        }
+        AVIF_CHECKRES(avifReadColorNclxProperty(&gainMapItemTmp->properties,
+                                                &gainMap->image->colorPrimaries,
+                                                &gainMap->image->transferCharacteristics,
+                                                &gainMap->image->matrixCoefficients,
+                                                &gainMap->image->yuvRange,
+                                                /*cicpSet=*/NULL));
     }
 
     // Only set the output parameters after everything has been validated.