Mode setting: only check channel user modes when on channel

Mode setting: only check channel user modes when on channel

Only check the channel user modes of the initiator if he is joined to
this channel and not an IRC operator enforcing modes (which requires
the configuration option "OperCanUseMode" to be enabled), because trying
to check channel user modes of a non-member results in this assertion:

 Assertion failed: (cl2chan != NULL), function Channel_UserModes,
  file channel.c, line 742.

This closes bug #147, thanks to James Kirwill <james.kirwill@bk.ru>
for tracking this down!