avcodec/vmdaudio: Check chunk counts to avoid integer overflow

Fixes: signed integer overflow: 4 * 538976288 cannot be represented in type 'int'
Fixes: 18622/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_VMDAUDIO_fuzzer-5092166174507008

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