Fixes: signed integer overflow: -1082982400 + -1079364728 cannot be represented in type 'int'
Fixes: 67910/clusterfuzz-testcase-minimized-ffmpeg_SWS_fuzzer-5329011971522560
The input is 9bit in 16bit, the fuzzer fills all 16bit thus generating "invalid" input
No overflow should happen with valid input.
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>