vp8: always update next_framep[] before returning from decode_frame().

Also slightly move around code not allocate a new frame if we won't
decode it. This prevents us from putting undecoded frames in frame
pointers, which (in mt decoding) other threads will use and wait on
as references, causing a deadlock (if we skipped decoding) or a crash
(if we didn't initialized next_framep[] at all).

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
