drm/radeon/kms: Fix oops after radeon_cs_parser_init() failure.

If radeon_cs_parser_init() fails, radeon_cs_ioctl() calls
radeon_cs_parser_fini() with the non-zero error value. The latter dereferenced
parser->ib which hasn't been initialized yet -> boom. Add a test for parser->ib
being non-NULL before dereferencing it.

Signed-off-by: Michel Dänzer <daenzer@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>