Fixes: runtime error: signed integer overflow: 2147483646 + 2048 cannot be represented in type 'int'
Fixes: 4479/clusterfuzz-testcase-minimized-6529894147162112

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