avformat/iamf_parse: consider nb_substreams when accessing substreams array

Fixes: out of array access
Fixes: 68584/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-6256656668229632

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

libavformat/iamf_parse.c

@@ -355,6 +355,9 @@ static int scalable_channel_layout_config(void *s, AVIOContext *pb,
         substream_count = avio_r8(pb);
         coupled_substream_count = avio_r8(pb);
 
+        if (substream_count + k > audio_element->nb_substreams)
+            return AVERROR_INVALIDDATA;
+
         audio_element->layers[i].substream_count         = substream_count;
         audio_element->layers[i].coupled_substream_count = coupled_substream_count;
         if (output_gain_is_present_flag) {