avifParseImageGridBox should consume all item data (#269)

At the end of avifParseImageGridBox(), there should be no remaining
bytes in the stream 's'.

src/read.c

diff --git a/src/read.c b/src/read.c
index 0c2f66c..2229475 100644
--- a/src/read.c
+++ b/src/read.c
@@ -986,7 +986,7 @@
     if (grid->outputWidth == 0 || grid->outputHeight == 0 || grid->outputWidth > AVIF_MAX_IMAGE_SIZE / grid->outputHeight) {
         return AVIF_FALSE;
     }
-    return AVIF_TRUE;
+    return avifROStreamRemainingBytes(&s) == 0;
 }
 
 static avifBool avifParseImageSpatialExtentsProperty(avifProperty * prop, const uint8_t * raw, size_t rawLen)