bug#37344: rcirc: nil gets interpreted as a nickname

[Naïm Favier]

Severity: normal

Ever since I changed my nickname to "nil" on Freenode, I've been
getting occasional private messages from unknown users consisting of a
single empty CTCP ACTION. After a bit of investigating, it turned out
they all used rcirc. The situation was clear at that point: somewhere
in rcirc's source code, a nil value is being implicitly converted to a
string and used as the target of a PRIVMSG command.

The bug seems to be reproducible by issuing "/me" (without arguments)
inside a server buffer: the "nil" user on that server gets sent an
empty ACTION.

Suggested fix: in rcirc-send-privmsg, fail if target is nil. It might
be useful to check other places where the "%s" format is used, to
discover similar bugs.