irc_channel_name_gen: handle g_convert_with_fallback failures
First fallback to ASCII without TRANSLIT, and if that fails too, just
give up by returning NULL.
Basically the same thing as 3a27896 (a netbsd specific fix), but for
channel names. This wasn't needed before because the older version of
this code caught the NULL from the ASCIITRANSLIT attempt and gave up
immediately, while the refactored version lacked null checking.
