Just do nothing if crct_set_base() is called with no FB.

The oops happens when the user switches between X & vt or in some case
when changing mode.
Signed-off-by: NJerome Glisse <jglisse@redhat.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NDave Airlie <airlied@redhat.com>