lavf/http: remove duplicated dec flag.
avcodec/mpegvideo_motion: Use a field from the current frame if the last is unavailable in DMV & 16x8

Fixes null pointer dereference

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