h264_refs: Check for attempts to assign pictures to short & long.

Fixes null pointer dereference

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
wmv2: Use emu edge mode when the edge is too small

Fixes out of array read

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
wmv2: drop non emu edge mode

slightly (~ 0.5%) faster, tested with time ./ffmpeg

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
gmc: Always use edge emu

The gmc functions access more pixels than there is allocated memory.
Fixes out of array reads

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