degesch: flush I/O buffers on disconnect

Discovered thanks to weercd.

degesch.c

@@ -3428,6 +3428,9 @@ irc_disconnect (struct server *s)
 	s->socket_event.closed = true;
 	poller_fd_reset (&s->socket_event);
 
+	str_reset (&s->read_buffer);
+	str_reset (&s->write_buffer);
+
 	// All of our timers have lost their meaning now
 	irc_cancel_timers (s);