Fixes: index 13 out of bounds for type 'const struct IAMFSoundSystemMap [13]'
Fixes: 67796/clusterfuzz-testcase-minimized-ffmpeg_dem_IAMF_fuzzer-4554553191104512

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