vscreens: resize: grow: last vscreen should be N-1 rather than N

growing the number (set vscreens N) at runtime would always grow one
extra: new set of vscreens had the last one indexed at N, rather than
N-1.  this meant there was always one extra when we grew the vscreen
count.  the end must be shifted by one left.

conversely, shrinking already worked correctly: the new set of vscreens
would have the last one indexed at N-1

vscreen.c

@@ -125,7 +125,7 @@ vscreens_resize(int n)
 		}
 	} else if (n > defaults.vscreens) {
 		list_for_each_entry(scr, &rp_screens, node) {
-			for (x = defaults.vscreens; x <= n; x++) {
+			for (x = defaults.vscreens; x < n; x++) {
 				cur = xmalloc(sizeof(rp_vscreen));
 				init_vscreen(cur, scr);
 				list_add_tail(&cur->node, &scr->vscreens);