avcodec/mpegvideo: Fix null pointer dereference of picture array

Fixes: 0d0a2bace067d09c08f0fa5340496c23_signal_sigsegv_7ffff713351a_342_WobblyWindowsIntro.avi with memlimit of 67108864

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