avcodec/cavsdec: Check sym_factor

Fixes: runtime error: signed integer overflow: 25984 * 130560 cannot be represented in type 'int'

Fixes: 1404/clusterfuzz-testcase-minimized-5000441286885376

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