avformat/iamf_parse: Check output_channel_count

Fixes: -nan is outside the range of representable values of type 'int'
Fixes: 377072730/clusterfuzz-testcase-minimized-ffmpeg_dem_IAMF_fuzzer-6545416570601472

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

libavformat/iamf_parse.c

@@ -422,7 +422,7 @@ static int ambisonics_config(void *s, AVIOContext *pb,
 
     output_channel_count = avio_r8(pb);  // C
     substream_count = avio_r8(pb);  // N
-    if (audio_element->nb_substreams != substream_count)
+    if (audio_element->nb_substreams != substream_count || output_channel_count == 0)
         return AVERROR_INVALIDDATA;
 
     order = floor(sqrt(output_channel_count - 1));