cursor-renderer/native: Fix Xcursor image loading error handling

Check the return value whether there was an error, and change to
g_warning to warn while at it.

https://gitlab.gnome.org/GNOME/mutter/issues/77

src/backends/meta-cursor-sprite-xcursor.c

@@ -149,10 +149,9 @@ load_from_current_xcursor_image (MetaCursorSpriteXcursor *sprite_xcursor)
                                            rowstride,
                                            (uint8_t *) xc_image->pixels,
                                            &error);
-
+  if (!texture)
-  if (error)
     {
-      meta_warning ("Failed to allocate cursor texture: %s\n", error->message);
+      g_warning ("Failed to allocate cursor texture: %s\n", error->message);
       cogl_error_free (error);
     }