fix intermittent crash in make check

The original crash was in GtkSalMenu::Active, triggered from
RefreshMenusUnity and it looked like the refresh method was accessing
already freed memory.

Change-Id: Ib07d839a976869eb8719e71e0653f1a72e3b0e80

vcl/unx/gtk/window/gtksalmenu.cxx

@@ -450,9 +450,18 @@ void GtkSalMenu::SetSubMenu( SalMenuItem* pSalMenuItem, SalMenu* pSubMenu, unsig
     pItem->mpSubMenu = pGtkSubMenu;
 }
 
+class GdkThreadLock
+{
+public:
+    GdkThreadLock() { gdk_threads_enter(); }
+    ~GdkThreadLock() { gdk_threads_leave(); }
+};
+
 static bool bInvalidMenus = false;
 static gboolean RefreshMenusUnity(gpointer)
 {
+    GdkThreadLock aLock;
+
     SalDisplay* pSalDisplay = GetGenericData()->GetSalDisplay();
     std::list< SalFrame* >::const_iterator pSalFrame = pSalDisplay->getFrames().begin();
     std::list< SalFrame* >::const_iterator pEndSalFrame = pSalDisplay->getFrames().end();