avcodec/dca_xll: Fix runtime error: signed integer overflow: -1073741824 * 32768 cannot be represented in type 'int'

Fixes: 629/clusterfuzz-testcase-6697457381539840

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