Allow cmd_mouse_at return arguments to be NULL.

cmd.c

@@ -601,7 +601,9 @@ cmd_mouse_at(struct window_pane *wp, struct mouse_event *m, u_int *xp,
 	if (y < wp->yoff || y >= wp->yoff + wp->sy)
 		return (-1);
 
+	if (xp != NULL)
 		*xp = x - wp->xoff;
+	if (yp != NULL)
 		*yp = y - wp->yoff;
 	return (0);
 }