avformat/alp: Check num_channels

Fixes: division by 0
Fixes: 45615/clusterfuzz-testcase-minimized-ffmpeg_dem_ALP_fuzzer-582660348405350
Fixes: 45625/clusterfuzz-testcase-minimized-ffmpeg_dem_ALP_fuzzer-4821437943250944
Fixes: 45839/clusterfuzz-testcase-minimized-ffmpeg_IO_DEMUXER_fuzzer-5857286998851584.fuzz
Fixes: 45844/clusterfuzz-testcase-minimized-ffmpeg_IO_DEMUXER_fuzzer-6552705772945408

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