validate_img,validate_hbd_input: fix crash w/monochrome

Use the monochrome setting from the encoder configuration to determine
whether to check the U/V planes. Previously the value from `aom_image_t`
was used. Not all applications may set this value.

Bug: 513603812
Change-Id: I7fd5d37f2ad863b6392d508f5ad542b68b9c7c88
(cherry picked from commit 13cbb78a1a9ebb5193ef8fae664a507efb4287e5)

av1/av1_cx_iface.c

diff --git a/av1/av1_cx_iface.c b/av1/av1_cx_iface.c
index 77e48de..44482f5 100644
--- a/av1/av1_cx_iface.c
+++ b/av1/av1_cx_iface.c
@@ -1025,9 +1025,10 @@
     const int max_val = 1 << bit_depth;
     // Note there is no high bitdepth version of NV12 defined. If one is
     // added, `num_planes` should be 2 in that case.
-    const int num_planes = img->monochrome ? 1 : 3;
+    const int num_planes = ctx->cfg.monochrome ? 1 : 3;
     for (int plane = 0; plane < num_planes; ++plane) {
       const unsigned short *src = (const unsigned short *)img->planes[plane];
+      if (!src) return AOM_CODEC_INVALID_PARAM;
       const unsigned int stride = img->stride[plane] / 2;
       const unsigned int ph = aom_img_plane_height(img, plane);
       const unsigned int pw = aom_img_plane_width(img, plane);

test/encode_api_test.cc

diff --git a/test/encode_api_test.cc b/test/encode_api_test.cc
index ba482bc..d6b3c8d 100644
--- a/test/encode_api_test.cc
+++ b/test/encode_api_test.cc
@@ -390,8 +390,10 @@
            ++check_input_range) {
         uint8_t *u = img->planes[AOM_PLANE_U];
         uint8_t *v = img->planes[AOM_PLANE_V];
-        img->monochrome = cfg.monochrome;
-        if (img->monochrome) {
+        // Applications may not set img->monochrome. Leave it set to 0, but
+        // clear the planes to ensure the encoder properly ignores their
+        // values.
+        if (cfg.monochrome) {
           img->planes[AOM_PLANE_U] = img->planes[AOM_PLANE_V] = nullptr;
         }