avcodec/alsdec: Limit maximum channels to 512

avcodec/alsdec: Limit maximum channels to 512

There seems to be no limit in the specification and upto 64k could be stored
512 is choosen as limit as thats the maximum in a conformance sample

An alternative to this patch would be a max_channels variable

Fixes: OOM
Fixes: 16200/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ALS_fuzzer-5764788793114624
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Suggested-by: Thilo Borgmann <thilo.borgmann@mail.de>
Reviewed-by: Thilo Borgmann <thilo.borgmann@mail.de>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>