vc1: Do not assume seek happens after decoding

If a seek is requested before the decoding start there is no
current picture.

CC: libav-stable@libav.org
(cherry picked from commit 3e348ecfc6ab1830e43288a9e12e8f0a000afbcb)
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>