avcodec/h264_ps: Fix runtime error: signed integer overflow: -1094995528 * 2 cannot be represented in type 'int'

Fixes: 615/clusterfuzz-testcase-5488002644049920
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
