h264: reset num_reorder_frames if it is invalid

An invalid VUI is not considered a fatal error, so the SPS containing it
may still be used. Leaving an invalid value of num_reorder_frames there
can result in writing over the bounds of H264Context.delayed_pic.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC:libav-stable@libav.org
