Test avifImageRGBToYUV() with upsamplingsIndex=0

Since avifImageRGBToYUV() ignores rgb.chromaUpsampling, we only need to
test avifImageRGBToYUV() with a single upsamplingsIndex.

tests/oss-fuzz/avif_decode_fuzzer.cc

diff --git a/tests/oss-fuzz/avif_decode_fuzzer.cc b/tests/oss-fuzz/avif_decode_fuzzer.cc
index 6455560..0be1cd7 100644
--- a/tests/oss-fuzz/avif_decode_fuzzer.cc
+++ b/tests/oss-fuzz/avif_decode_fuzzer.cc
@@ -48,7 +48,9 @@
                             rgb.chromaUpsampling = upsamplings[upsamplingsIndex];
                             avifRGBImageAllocatePixels(&rgb);
                             avifResult rgbResult = avifImageYUVToRGB(decoder->image, &rgb);
-                            if (rgbResult == AVIF_RESULT_OK) {
+                            // Since avifImageRGBToYUV() ignores rgb.chromaUpsampling, we only need
+                            // to test avifImageRGBToYUV() with a single upsamplingsIndex.
+                            if ((rgbResult == AVIF_RESULT_OK) && (upsamplingsIndex == 0)) {
                                 for (size_t yuvDepthsIndex = 0; yuvDepthsIndex < yuvDepthsCount; ++yuvDepthsIndex) {
                                     // ... and back to YUV
                                     avifImage * tempImage = avifImageCreate(decoder->image->width,