Fix crash with /upgrade (problem when closing buffers)

src/core/wee-command.c

@@ -3247,6 +3247,8 @@ command_upgrade (void *data, struct t_gui_buffer *buffer,
     exec_args[0] = ptr_binary;
     exec_args[3] = strdup (weechat_home);
     
+    weechat_quit = 1;
+    
     /* save layout, unload plugins, save config, then upgrade */
     gui_layout_save_on_exit ();
     plugin_end ();

src/gui/gui-buffer.c

@@ -1404,7 +1404,7 @@ gui_buffer_close (struct t_gui_buffer *buffer)
            window is displaying buffer, or buffer # - 1 */
         ptr_buffer_visited = NULL;
         if (CONFIG_BOOLEAN(config_look_jump_previous_buffer_when_closing)
-            && (gui_current_window->buffer == buffer))
+            && gui_current_window && (gui_current_window->buffer == buffer))
         {
             index = gui_buffer_visited_get_index_previous ();
             if (index >= 0)