error_concealment: avoid using the picture if not fully setup

Fixes state becoming inconsistent and a null pointer dereference.

CC: libav-stable@libav.org
Bug-Id: CVE-2013-0860
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
Signed-off-by: Anton Khirnov <anton@khirnov.net>