avcodec/dcadsp: Fix runtime error: signed integer overflow: 394625024 * 8 cannot be represented in type 'int'

Fixes: 643/clusterfuzz-testcase-5209078743695360

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