Fix a memory leak in GtkCSSProvider

Found by Felix Riemann
https://bugzilla.gnome.org/show_bug.cgi?id=641558

gtk/gtkcssprovider.c

@@ -4232,6 +4232,8 @@ gtk_css_provider_get_named (const gchar *name,
             }
           else
             g_hash_table_insert (themes, g_strdup (key), provider);
+
+          g_free (path);
         }
     }