Do nothing in vt_upgrade if there is no vt driver

Previously, if no drivers attached at boot we would panic with
"vtbuf_fill_locked begin.tp_row 0 must be < screen height 0".

PR:		192248
Reviewed by:	ray
MFC after:	3 days
Sponsored by:	DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D954

sys/dev/vt/vt_core.c

@@ -2498,6 +2498,8 @@ vt_upgrade(struct vt_device *vd)
 
 	if (!vty_enabled(VTY_VT))
 		return;
+	if (main_vd->vd_driver == NULL)
+		return;
 
 	for (i = 0; i < VT_MAXWINDOWS; i++) {
 		vw = vd->vd_windows[i];