libview: Fix goto window in presentation mode

It wasn't correctly set as modal.

origin commit:
https://git.gnome.org/browse/evince/commit/?h=gnome-3-2&id=987aab6

libview/ev-view-presentation.c

@@ -668,9 +668,9 @@ ev_view_presentation_goto_window_create (EvViewPresentation *pview)
 	GtkWindow *toplevel, *goto_window;
 
 	toplevel = GTK_WINDOW (gtk_widget_get_toplevel (GTK_WIDGET (pview)));
-	goto_window = GTK_WINDOW (pview->goto_window);
 
 	if (pview->goto_window) {
+	    goto_window = GTK_WINDOW (pview->goto_window);
 		if (gtk_window_has_group (toplevel))
 			gtk_window_group_add_window (gtk_window_get_group (toplevel), goto_window);
 		else if (gtk_window_has_group (goto_window))
@@ -680,6 +680,7 @@ ev_view_presentation_goto_window_create (EvViewPresentation *pview)
 	}
 
 	pview->goto_window = gtk_window_new (GTK_WINDOW_POPUP);
+	goto_window = GTK_WINDOW (pview->goto_window);
 	gtk_window_set_screen (goto_window, gtk_widget_get_screen (GTK_WIDGET (pview)));
 
 	if (gtk_window_has_group (toplevel))