Fix assertion when uniniting without any notifications

The lazy initialization from commit 3dc04fa caused a small regression: When a
process would init and uninit libnotify without ever sending a notification,
_proxy was still NULL, which caused an assertion

(process:20953): GLib-GObject-CRITICAL **: g_object_unref: assertion `G_IS_OBJECT (object)' failed

Bug-Ubuntu: https://bugs.launchpad.net/bugs/570462
(cherry picked from commit 1a103f1eee)

libnotify/notify.c

@@ -139,7 +139,9 @@ notify_uninit (void)
                 }
         }
 
-        g_object_unref (_proxy);
+        if (_proxy != NULL) {
+            g_object_unref (_proxy);
+        }
 
         _initted = FALSE;
 }