tdf#128241: fix crash when opening print dialog (gtk3 only)

See bt here:
https://bugs.documentfoundation.org/attachment.cgi?id=155131

Change-Id: Ibd220f6f41b6cee6217465c534fac1af4bb2b51b
Reviewed-on: https://gerrit.libreoffice.org/81123
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>
diff --git a/vcl/unx/gtk3/gtk3gtkinst.cxx b/vcl/unx/gtk3/gtk3gtkinst.cxx
index 5837e00..0d8f4d3 100644
--- a/vcl/unx/gtk3/gtk3gtkinst.cxx
+++ b/vcl/unx/gtk3/gtk3gtkinst.cxx
@@ -5349,7 +5349,7 @@ private:
        {
            const GtkWidget* pTabWidget = gtk_notebook_get_tab_label(pNotebook, gtk_notebook_get_nth_page(pNotebook, i));
            const gchar* pStr = gtk_buildable_get_name(GTK_BUILDABLE(pTabWidget));
            if (strcmp(pStr, rIdent.getStr()) == 0)
            if (pStr && strcmp(pStr, rIdent.getStr()) == 0)
                return i;
        }
        return -1;