vc1dec: Make sure last_picture is initialized in vc1_decode_skip_blocks

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>
(cherry picked from commit 5e25fdbfe01635cfc650ac4adc27d434b2df0d64)
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

Conflicts:
	libavcodec/vc1dec.c
(cherry picked from commit 494f2d4f9e834db1eaf1a7d0160d497f9802013d)
vc1dec: Don't decode slices when the latest slice header failed to decode

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>

Conflicts:
	libavcodec/vc1dec.c