Fixes runtime error: signed integer overflow: 1997494407 + 613252359 cannot be represented in type 'int'
Fixes: 2014/clusterfuzz-testcase-minimized-5186337030275072

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