Fixes: signed integer overflow: -1027919784 + -1120041624 cannot be represented in type 'int'
Fixes: 15406/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_WMV3_fuzzer-5700646528876544

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