gobject: avoid calling g_weak_ref_set() when intializing GWeakRef to NULL

If object is NULL, g_weak_ref_set() really does nothing. However,
it will pointlessly take a lock, before deciding to do nothing.
Avoid that.

gobject/gobject.c

@@ -5001,6 +5001,7 @@ g_weak_ref_init (GWeakRef *weak_ref,
 {
   weak_ref->priv.p = NULL;
 
+  if (object)
     g_weak_ref_set (weak_ref, object);
 }