Update size in vt_grow if we don't have to reallocate

Update size in vt_grow if we don't have to reallocate

vt_grow may be called with a new size that's larger than previous but
does not require reallocation - for example, when the number of columns
is the same and new number of rows is less than the history size.
Prior to this change we would fail to update vb_scr_size, and then hit
a KASSERT when trying to write to the newly visible rows.

Sponsored by:	The FreeBSD Foundation