avcodec/dca_xll: signed integer overflow: 255251 * 32768 cannot be represented in type 'int'

Fixes: 627/clusterfuzz-testcase-5020897033322496
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
