plugin XEP-0045: fixed crash when muc service is not found

sat/plugins/plugin_xep_0045.py

@@ -362,6 +362,8 @@ class XEP_0045(object):
                 # use "conference/text" identity (instead of "conference/irc")
                 muc_service = service
                 break
+        else:
+            muc_service = None
         defer.returnValue(muc_service)
 
     def _getUniqueName(self, muc_service="", profile_key=C.PROF_KEY_NONE):