Fix AvifDecodeTest.NonPersistentIOBug506387278

AvifDecodeTest.NonPersistentIOBug506387278 fails when libaom or libgav1
is used as the AV1 decoder, because the test input file
poc_b_506387278.avif contains errors.

The first error is easier to fix. The two padding OBUs do not contain
trailing bits. This is fixed with the following Python script:

```
with open("tests/data/poc_b_506387278.avif", "r+b") as f:
  f.seek(0x1702)
  f.write(b'\x80')
  f.seek(0x559a)
  f.write(b'\x80')
```

Note: The two padding OBUs can be identified by searching for the byte
0x7a, which is the OBU header for a padding OBU. The OBU sizes for the
two padding OBUs are 0xbc 0x26 and 0xf0 0x7c, respectively, when encoded
in leb128().

The second error is harder to fix. The data for the first image contains
partial frame data for the second frame. The extra incomplete frame data
makes avifDecoderNextImage() fail when libaom or libgav1 is used as the
AV1 decoder. Since this test is originally intended for CrabbyAvif and
dav1d, in libavif it is used as a fuzz test, so we can relax the
expected results of the avifDecoderNextImage() calls in the test.

tests/data/poc_b_506387278.avif

diff --git a/tests/data/poc_b_506387278.avif b/tests/data/poc_b_506387278.avif
index 66b4380..e5b2ec3 100644
--- a/tests/data/poc_b_506387278.avif
+++ b/tests/data/poc_b_506387278.avif
Binary files differ

tests/gtest/avifdecodetest.cc

diff --git a/tests/gtest/avifdecodetest.cc b/tests/gtest/avifdecodetest.cc
index afde48d..2cb6ba0 100644
--- a/tests/gtest/avifdecodetest.cc
+++ b/tests/gtest/avifdecodetest.cc
@@ -175,8 +175,18 @@
   EXPECT_EQ(decoder->imageSequenceTrackPresent, AVIF_TRUE);
   EXPECT_EQ(decoder->imageCount, 2);
   if (testutil::Av1DecoderAvailable()) {
-    EXPECT_EQ(avifDecoderNextImage(decoder.get()), AVIF_RESULT_OK);
-    EXPECT_EQ(avifDecoderNextImage(decoder.get()), AVIF_RESULT_OK);
+    avifResult result = avifDecoderNextImage(decoder.get());
+    if (result != AVIF_RESULT_OK) {
+      EXPECT_EQ(result, AVIF_RESULT_DECODE_COLOR_FAILED);
+      return;
+    }
+    result = avifDecoderNextImage(decoder.get());
+    if (result != AVIF_RESULT_OK) {
+      EXPECT_EQ(result, AVIF_RESULT_DECODE_COLOR_FAILED);
+      return;
+    }
+    EXPECT_EQ(avifDecoderNextImage(decoder.get()),
+              AVIF_RESULT_NO_IMAGES_REMAINING);
   }
 }