Prevent errors on first invocation

The group buffer creation code in `rcirc-groups:list-conversations` was
incorrect and ended up calling `with-current-buffer` using `nil` as the
value for the buffer name when handling the first IRC message, causing
an "(wrong-type-argument stringp nil)" error.
Merge pull request #3 from fgallina/first-handle-fix

Prevent errors on first invocation