avformat/iamfdec: Check nb_layers before dereferencing layer

Fixes: dereferencing pointers near NULL
Fixes: 70432/clusterfuzz-testcase-minimized-ffmpeg_dem_IAMF_fuzzer-5255672845893632
Fixes: 70877/clusterfuzz-testcase-minimized-ffmpeg_dem_IAMF_fuzzer-5348547432611840

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/iamfdec.c

@@ -107,7 +107,7 @@ static int iamf_read_header(AVFormatContext *s)
             if (ret < 0)
                 return ret;
 
-            if (!i && !j && audio_element->layers[0].substream_count == 1)
+            if (!i && !j && audio_element->nb_layers && audio_element->layers[0].substream_count == 1)
                 st->disposition |= AV_DISPOSITION_DEFAULT;
             else
                 st->disposition |= AV_DISPOSITION_DEPENDENT;