avcodec/h264_refs: set last_pic_for_ec only if it has not been set previously

This ensures we do not loose the frame in case or multiple clears
Fixes out of array read
Fixes: asan_heap-oob_2fa47ea_2100_cov_1278768963_ff_add_pixels_clamped_mmx.m2ts

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