avcodec/h264_direct: Fix runtime error: signed integer overflow: -9 - 2147483647 cannot be represented in type 'int'

Fixes: 864/clusterfuzz-testcase-4774385942528000

See: [FFmpeg-devel] [PATCH 1/2] avcodec/h264_direct: Fix runtime error: signed integer overflow: 2147483647 - -14133 cannot be represented in type 'int'
See: [FFmpeg-devel] [PATCH 2/2] avcodec/h264_direct: Fix runtime error: signed integer overflow: -9 - 2147483647 cannot be represented in type 'int'

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