avcodec/h264_slice: Fix overflow in slice offset

Fixes: runtime error: signed integer overflow: 1610612736 * 2 cannot be represented in type 'int'
Fixes: 2817/clusterfuzz-testcase-minimized-5289691240726528

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