ts6: don't crash when CHGHOST target is a nick instead of UID

(cherry picked from commit 4dcbc85a81)

protocols/ts6.py

@@ -630,7 +630,7 @@ class TS6Protocol(TS6BaseProtocol):
 
     def handle_chghost(self, numeric, command, args):
         """Handles incoming CHGHOST commands."""
-        target = args[0]
+        target = self._getUid(args[0])
         self.irc.users[target].host = newhost = args[1]
         return {'target': target, 'newhost': newhost}