Fixes: signed integer overflow: -2072 + -2147483646 cannot be represented in type 'int'
Fixes: 6097/clusterfuzz-testcase-minimized-5034145253163008

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