avcodec/dca: Fix multiple runtime error: signed integer overflow

Fixes: 680/clusterfuzz-testcase-5416627266912256
Fixes: 681/clusterfuzz-testcase-5013323462475776

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