zink: do not dereference NULL pointer
If first_frame_done isn't set, but fence is NULL, we end up dereferncing that NULL-pointer.
This can happen in the case where the first submitted batch has no work, and pfence was passed as a NULL-pointer.
While we're at it, simplify the check with the surrounding code, which also checks for a NULL-pointer here.
Fixes: e93ca92d ("zink: force explicit fence only on first frame flush")