Fixed some bugs.

Fix in interpreter: "xcommand" would be interpreted as "/command" whenever "command" was a legal command, no matter what the "x" character was.

Fix in init.scm: Trying to send messages with diacritics (such as "ž") resulted in a crash. Initializing the locale fixes that.

Fix in roster: ft_roster_lookup would lead to weird behaviour when called with empty string (""). This is due to the fact that the result of g_strsplit on "" is an empty vector (a special case: see https://developer.gnome.org/glib/stable/glib-String-Utility-Functions.html#g-strsplit). I found this bug while trying to define my own command to clear the current buddy using (ft-set-current-buddy! "").