Because the read handler may discover that the connection has been closed,
and then remove the socket from the poller. It that case it is no longer
valid to try to call the write handler (which may try to reconnect, but
since that socket is no longer managed, this is not OK).

Explain that the behaviour is otherwise undefined, in the comment.

fix #2524

Since “!” is also the separator between the nickname and the user hostname,
having “!” as the user mode (e.g. !nick!~some@host.bla) would cause the nick
to be empty. Now we skip it if it is a valid user mode indicator.

fix #2496