desktop-shell: fix resize grab related crash

This crash was happening when *releasing* a pointer button on a window
that was being resized and got destroyed during the grab.

[@daniels: Cosmetic fixes; apply same fix to grab cancel.]

desktop-shell/shell.c

@@ -1654,12 +1654,16 @@ resize_grab_button(struct weston_pointer_grab *grab,
 	struct weston_resize_grab *resize = (struct weston_resize_grab *) grab;
 	struct weston_pointer *pointer = grab->pointer;
 	enum wl_pointer_button_state state = state_w;
-	struct weston_desktop_surface *desktop_surface =
-		resize->base.shsurf->desktop_surface;
 
 	if (pointer->button_count == 0 &&
 	    state == WL_POINTER_BUTTON_STATE_RELEASED) {
-		weston_desktop_surface_set_resizing(desktop_surface, false);
+		if (resize->base.shsurf != NULL) {
+			struct weston_desktop_surface *desktop_surface =
+				resize->base.shsurf->desktop_surface;
+			weston_desktop_surface_set_resizing(desktop_surface,
+							    false);
+		}
+
 		shell_grab_end(&resize->base);
 		free(grab);
 	}
@@ -1669,10 +1673,13 @@ static void
 resize_grab_cancel(struct weston_pointer_grab *grab)
 {
 	struct weston_resize_grab *resize = (struct weston_resize_grab *) grab;
+
+	if (resize->base.shsurf != NULL) {
 		struct weston_desktop_surface *desktop_surface =
 			resize->base.shsurf->desktop_surface;
-
 		weston_desktop_surface_set_resizing(desktop_surface, false);
+	}
+
 	shell_grab_end(&resize->base);
 	free(grab);
 }