Fix for a crash, when "client menu" is placed in "root menu"

[JanisMancevics]

I noticed that if client-menu is added as entry in root-menu openbox will crash if there isn't an active client.

I added this in "openbox/menuframe.c" to fix it.

@@ -1390,6 +1390,11 @@ void menu_frame_select_next(ObMenuFrame *self)
 {
     GList *it = NULL, *start;
 
+    if (self == NULL)
+    {
+        return;
+    }
+    
     if (self->entries) {
         start = it = g_list_find(self->entries, self->selected);
         while (TRUE) {