avcodec/h264_slice: Fix signed integer overflow

Fixes: runtime error: signed integer overflow: 26 + 2147483644 cannot be represented in type 'int'
Fixes: 2456/clusterfuzz-testcase-minimized-4822695051001856

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