missing g_signal_handler_disconnect in GtkInstanceWidget

found by loplugin:unusedfields

Change-Id: Iefd64e879b32f696456c51da595e462295e5c58e
Reviewed-on: https://gerrit.libreoffice.org/64820
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

vcl/unx/gtk3/gtk3gtkinst.cxx

@@ -1606,6 +1606,8 @@ public:
     {
         if (m_nKeyPressSignalId)
             g_signal_handler_disconnect(m_pWidget, m_nKeyPressSignalId);
+        if (m_nKeyReleaseSignalId)
+            g_signal_handler_disconnect(m_pWidget, m_nKeyReleaseSignalId);
         if (m_nFocusInSignalId)
             g_signal_handler_disconnect(m_pWidget, m_nFocusInSignalId);
         if (m_nFocusOutSignalId)