Update xchat_otr.c

Fixed http://irc-otr.tuxfamily.org/bugs/show_bug.cgi?id=9
messages directed at someone else (most likely a channel you and the sender have joined) will not get consumed by OTR.
Disclaimer: I don't have a clue about IRC and whether the word indexing is identical with different implementations (e.g. IRC prefixes), so this might or might not work.