Fixes: runtime error: signed integer overflow: -2147483647 - 2 cannot be represented in type 'int'
Fixes: 2702/clusterfuzz-testcase-minimized-4511932591636480

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 74c1c22d)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>