avcodec/vp8: Check buffer size in vp8_decode_frame_header()

avoids null pointer dereference
Fixes: signal_sigsegv_d5de40_964_vp80-00-comprehensive-010.ivf with memlimit of 1048576

Found-by: Samuel Groß, Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
