cogl: Plug leak in error path

If the renderer fails to connect, we were leaking the renderer.

CID: #1505882
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2061>

cogl/cogl/cogl-context.c

@@ -146,6 +146,7 @@ cogl_context_new (CoglDisplay *display,
       CoglRenderer *renderer = cogl_renderer_new ();
       if (!cogl_renderer_connect (renderer, error))
         {
+          cogl_object_unref (renderer);
           g_free (context);
           return NULL;
         }