Fix segmentation fault on IRC server reply

When Pidgin received `:nick!user@host JOIN #channel` from an IRC
server, it worked fine, but when it received
`:nick JOIN #channel`, it crashed with a segmentation fault.

Testing Done:
Tested with a custom IRC server that only sends the nickname. Also tested on Libera Chat.

Bugs closed: PIDGIN-17375

Reviewed at https://reviews.imfreedom.org/r/1484/