gtkentrycompletion: fix a critical warning

There are early returns in this method before the completion timeout
is set later on, so set the source to 0 to avoid trying to remove it
later again.

https://bugzilla.gnome.org/show_bug.cgi?id=725824

gtk/gtkentrycompletion.c

@@ -2491,7 +2491,10 @@ gtk_entry_completion_changed (GtkWidget *widget,
 
   /* (re)install completion timeout */
   if (completion->priv->completion_timeout)
+  {
     g_source_remove (completion->priv->completion_timeout);
+    completion->priv->completion_timeout = 0;
+  }
 
   if (!gtk_entry_get_text (entry))
     return;