avcodec/h264_ps: Fix runtime error: signed integer overflow: 2147483647 + 26 cannot be represented in type 'int'

Fixes: 902/clusterfuzz-testcase-4561155144024064

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
Reviewed-by: "Ronald S. Bultje" <rsbultje@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
