K 10
svn:author
V 6
emaste
K 8
svn:date
V 27
2014-10-17T13:55:45.905434Z
K 7
svn:log
V 306
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

END