Terminate loop properly when cursor window is full.

The existing code exits the column-filling loop immediately but
may continue to try to fill rows.  Ordinarily this should fail
too but it's possible for the process to continue and then
blow up a little later.  Fixed this problem by exiting the row-
filling loop promptly.

Bug: 21406130
Change-Id: Ifab59188d5d1b40f3c1f5c97604dd48da71bf37b