Fixes: signed integer overflow: -293650 * 16525 cannot be represented in type 'int'
Fixes: 408304111/clusterfuzz-testcase-minimized-ffmpeg_SWS_fuzzer-4762210299871232
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>