vt(4): Don't recalculate buffer size if we don't know screen size

MF10:	r273037
MFC:	r272537

When the screen size is unknown, it's set to 0x0. We can't use that as
the buffer size, otherwise, functions such as vtbuf_fill() will fail.

This fixes a panic on RaspberryPi, where there's no vt(4) backend
configured early in boot.

PR:		193981
Tested by:	danilo@
Approved by:	re (marius)