Fixes: signed integer overflow: 65792 * 65312 cannot be represented in type 'int'
Fixes: 67819/clusterfuzz-testcase-minimized-ffmpeg_dem_WADY_fuzzer-5236100912185344

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