desktop-shell: Use the grabbed pointer in popup_grab_button

This should be identical to the pointer in shset->seat.

A later patch prevents direct access to seat->pointer, using the
known valid pointer in the grab will be nicer than using the
getter functions that patch introduces.

Reviewed-by: Jonas Ådahl <jadahl@gmail.com>
Signed-off-by: Derek Foreman <derekf@osg.samsung.com>

desktop-shell/shell.c

@@ -3233,7 +3233,7 @@ popup_grab_button(struct weston_pointer_grab *grab,
 		}
 	} else if (state == WL_POINTER_BUTTON_STATE_RELEASED &&
 		   (shseat->popup_grab.initial_up ||
-		    time - shseat->seat->pointer->grab_time > 500)) {
+		    time - grab->pointer->grab_time > 500)) {
 		popup_grab_end(grab->pointer);
 	}