Widelands host crashes if a client connection breaks

Just for the record, as this bug would surely be posted in future.

In build17 the host of a Widelands game crashes, if the connection to one client is suddenly lost (crash on client side, kill -9 widelands, etc.)
This is because the host notices that the client is gone and directly afterwards tries to send a message "client x left" to all clients - including the client that is already gone and which socket is thus invalid.

This bug was fixed in bzr rev. 6360