avcodec/mpeg4videodec: Check P cbpy

Fixes undefined behavior
Fixes: signal_sigsegv_c3097a_991_xtrem_e2_m64q15_a32sxx.3gp

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