avcodec/wmaprodec: Check that the streams channels do not exceed the overall channels

avcodec/wmaprodec: Check that the streams channels do not exceed the overall channels

Fixes: NULL pointer dereference
Fixes: 18075/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_XMA1_fuzzer-5708262036471808
Fixes: 18087/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_XMA1_fuzzer-5740627634946048
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>