avcodec/mpegvideo: Clear pointers in ff_mpv_common_init()

This ensures that no stale pointers leak through on any path

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 <michaelni@gmx.at>
