ffv1dec: check that global parameters do not change in version 0/1

Such changes are neither allowed nor supported

Found-by: ami_stuff
Bug-Id: CVE-2013-7020
CC: libav-stable@libav.org
Signed-off-by: Anton Khirnov <anton@khirnov.net>
(cherry picked from commit da7d839a0d3ec40423a665dc85e0cfaed3f92eb8)
Signed-off-by: Anton Khirnov <anton@khirnov.net>

Conflicts:
	libavcodec/ffv1dec.c