wayland: Check the relative pointer handle before destroying

If the relative protocol is unsupported, this will always be null and the destroy function won't be called.

(cherry picked from commit 0e87b71d08c75af8c4b02808038825d3b16bf854)

src/video/wayland/SDL_waylandevents.c

@@ -2668,8 +2668,10 @@ int Wayland_input_unlock_pointer(struct SDL_WaylandInput *input)
         w->locked_pointer = NULL;
     }
 
-    zwp_relative_pointer_v1_destroy(input->relative_pointer);
-    input->relative_pointer = NULL;
+    if (input->relative_pointer) {
+        zwp_relative_pointer_v1_destroy(input->relative_pointer);
+        input->relative_pointer = NULL;
+    }
 
     d->relative_mouse_mode = 0;