Ignore non-slack channels for nick_completion_cb too
Ignore non-slack channels for nick_completion_cb too

Failing to ignore this leads to harmless, but annoying tracebacks when
completing nicks in non-Slack channels.

rawdigits/wee-slack,trygveaa/wee-slack,wee-slack/wee-slack,scharfie/wee-slack,megalithic/wee-slack