Call bufferevent_free before closing file descriptor associated with it

or bugs in $EventMechanism on $OtherOS makes libevent get it's knickers
in a twist. From Dylan Alex Simon.

server-fn.c

@@ -337,8 +337,8 @@ server_destroy_pane(struct window_pane *wp)
 
 	old_fd = wp->fd;
 	if (wp->fd != -1) {
-		close(wp->fd);
 		bufferevent_free(wp->event);
+		close(wp->fd);
 		wp->fd = -1;
 	}