mpegvideo: synchronize AVFrame pointers in ERContext fully

Since error resilience uses AVFrame pointers instead of references it
has to copy NULL pointers too. After a codec flush the last/next frame
pointers in MpegEncContext are NULL and the old pointers remaining in
ERContext are invalid. Fixes a crash in vlc for android thumbnailer.
Reported and debugged by Adrien Maglo <magsoft@videolan.org>.
h263enc: keep block_last_index always valid during advanced intra coding

Prevents a triggered assert during fate-vsynth{1,2}-rv20 in
dct_unquantize_h263_intra_c().