avcodec/h264: Clear last_pic_for_ec on seeks and reinits

Fixes out of array read
Fixes: asan_heap-oob_2ff30d2_3479_cov_3803648058_src13_hrc7_525_420_2.264

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
