coverity#1351758 Uninitialized pointer field

Change-Id: I50639f0f83c2f1135a0c2002145e02fdf15034a5

vcl/inc/unx/gtk/gtkinst.hxx

@@ -137,7 +137,12 @@ class GtkDragSource : public cppu::WeakComponentImplHelper<css::datatransfer::dn
     css::uno::Reference<css::datatransfer::XTransferable> m_xTrans;
     VclToGtkHelper m_aConversionHelper;
 public:
-    GtkDragSource() : WeakComponentImplHelper( m_aMutex ) {}
+    GtkDragSource()
+        : WeakComponentImplHelper(m_aMutex)
+        , m_pFrame(nullptr)
+    {
+    }
+
     virtual ~GtkDragSource();
 
     // XDragSource