Add a unit test for issue #2979

Based on the PoC in https://github.com/AOMediaCodec/libavif/issues/2979.

tests/gtest/avifaltrtest.cc

diff --git a/tests/gtest/avifaltrtest.cc b/tests/gtest/avifaltrtest.cc
index 2889504..f1c1a01 100644
--- a/tests/gtest/avifaltrtest.cc
+++ b/tests/gtest/avifaltrtest.cc
@@ -70,6 +70,20 @@
   EXPECT_EQ(decoder->image->depth, depth);
 }
 
+// Verifies the fix for https://github.com/AOMediaCodec/libavif/issues/2979.
+TEST(AltrTest, ZeroImageContentToDecode) {
+  const std::string file_path =
+      std::string(data_path) + "weld_sato_12B_8B_q0.avif";
+
+  DecoderPtr decoder(avifDecoderCreate());
+  ASSERT_NE(decoder, nullptr);
+  decoder->imageContentToDecode = AVIF_IMAGE_CONTENT_NONE;
+  ImagePtr image(avifImageCreateEmpty());
+  ASSERT_NE(image, nullptr);
+  ASSERT_EQ(avifDecoderReadFile(decoder.get(), image.get(), file_path.c_str()),
+            AVIF_RESULT_NO_CONTENT);
+}
+
 //------------------------------------------------------------------------------
 
 }  // namespace