data-device: Clean up the logic in start_drag

Previoiusly, we had a mess of logic that was repeated with one of the
repeats negated.  Not only was this unnecisaraly confusing, but it
segfaulted and one of the negations was wrong.  This cleans the whole mess
up and should fix bug #79725.

src/data-device.c

@@ -646,17 +646,25 @@ data_device_start_drag(struct wl_client *client, struct wl_resource *resource,
 		       struct wl_resource *icon_resource, uint32_t serial)
 {
 	struct weston_seat *seat = wl_resource_get_user_data(resource);
+	struct weston_surface *origin = wl_resource_get_user_data(origin_resource);
 	struct weston_data_source *source = NULL;
 	struct weston_surface *icon = NULL;
+	int is_pointer_grab, is_touch_grab;
 	int32_t ret = 0;
 
-	if ((seat->pointer->button_count == 0 ||
-	    seat->pointer->grab_serial != serial ||
-	    !seat->pointer->focus ||
-	    seat->pointer->focus->surface != wl_resource_get_user_data(origin_resource)) &&
-		(seat->touch->grab_serial != serial ||
-		!seat->touch->focus ||
-		seat->touch->focus->surface != wl_resource_get_user_data(origin_resource)))
+	is_pointer_grab = seat->pointer &&
+			  seat->pointer->button_count == 1 &&
+			  seat->pointer->grab_serial == serial &&
+			  seat->pointer->focus &&
+			  seat->pointer->focus->surface == origin;
+
+	is_touch_grab = seat->touch &&
+			seat->touch->num_tp == 1 &&
+			seat->touch->grab_serial == serial &&
+			seat->touch->focus &&
+			seat->touch->focus->surface == origin;
+
+	if (!is_pointer_grab && !is_touch_grab)
 		return;
 
 	/* FIXME: Check that the data source type array isn't empty. */
@@ -672,14 +680,9 @@ data_device_start_drag(struct wl_client *client, struct wl_resource *resource,
 		return;
 	}
 
-	if (seat->pointer->button_count == 1 &&
-		seat->pointer->grab_serial == serial &&
-		seat->pointer->focus &&
-		seat->pointer->focus->surface == wl_resource_get_user_data(origin_resource))
+	if (is_pointer_grab)
 		ret = weston_pointer_start_drag(seat->pointer, source, icon, client);
-	else if (seat->touch->grab_serial != serial ||
-		seat->touch->focus ||
-		seat->touch->focus->surface != wl_resource_get_user_data(origin_resource))
+	else if (is_touch_grab)
 		ret = weston_touch_start_drag(seat->touch, source, icon, client);
 
 	if (ret < 0)