Correct vt(4) border calculations on font switch

If a vt(4) font does not exactly fit the screen dimensions, the console
window is offset so that it is centered.  A rectangle is drawn at the
top, left, right, and bottom of the screen, to erase any leftovers that
are outside of the new usable console area.

If the x offset or y offset is 0 then the left border or top border
respectively is not drawn.  The right and bottom borders may be one
pixel larger than necessary due to rounding, and are always drawn.

Prior to this change a 0 offset would result in a panic when calling
vt_drawrect with an x or y coordinate of -1.

Sponsored by:	The FreeBSD Foundation