Fix crashes when plugins modify UI during Close Context

This probably doesn't catch them all but setting tab color
and printing text at least do not crash in the common case.

src/fe-gtk/maingui.c

@@ -173,7 +173,7 @@ void
 fe_set_tab_color (struct session *sess, int col)
 {
 	struct session *server_sess = sess->server->server_session;
-	if (sess->gui->is_tab && (col == 0 || sess != current_tab))
+	if (sess->res->tab && sess->gui->is_tab && (col == 0 || sess != current_tab))
 	{
 		switch (col)
 		{
@@ -1010,12 +1010,6 @@ static void
 mg_topdestroy_cb (GtkWidget *win, session *sess)
 {
 /*	printf("enter mg_topdestroy. sess %p was destroyed\n", sess);*/
-
-	/* kill the text buffer */
-	gtk_xtext_buffer_free (sess->res->buffer);
-	/* kill the user list */
-	g_object_unref (G_OBJECT (sess->res->user_model));
-
 	session_free (sess);	/* tell hexchat.c about it */
 }
 
@@ -1026,11 +1020,6 @@ mg_ircdestroy (session *sess)
 {
 	GSList *list;
 
-	/* kill the text buffer */
-	gtk_xtext_buffer_free (sess->res->buffer);
-	/* kill the user list */
-	g_object_unref (G_OBJECT (sess->res->user_model));
-
 	session_free (sess);	/* tell hexchat.c about it */
 
 	if (mg_gui == NULL)
@@ -1092,7 +1081,10 @@ mg_tab_close (session *sess)
 	int i;
 
 	if (chan_remove (sess->res->tab, FALSE))
+	{
+		sess->res->tab = NULL;
 		mg_ircdestroy (sess);
+	}
 	else
 	{
 		for (i = 0, list = sess_list; list; list = list->next)
@@ -3653,6 +3645,9 @@ fe_server_callback (server *serv)
 void
 fe_session_callback (session *sess)
 {
+	gtk_xtext_buffer_free (sess->res->buffer);
+	g_object_unref (G_OBJECT (sess->res->user_model));
+
 	if (sess->res->banlist && sess->res->banlist->window)
 		mg_close_gen (NULL, sess->res->banlist->window);