avcodec/alac: Check decorr_shift to avoid invalid shift

avcodec/alac: Check decorr_shift to avoid invalid shift

Later the decorrelate_stereo call is guarded by channels == 2
and non-zero decorr_left_weight. Make sure decorr_shift is in
the expected shift range for that case.

Fixes: shift exponent 128 is too large for 32-bit type 'int'
Fixes: 23860/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ALAC_fuzzer-5751138914402304
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Alexander Strasser <eclipse7@gmx.net>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>