Don't attempt authentication if TURN is disabled

Explicitly disable authentication if TURN is disabled, as STUN is always
offered without authentication.  This avoids a failure if clients try to
authenticate while TURN is disabled (due to 'auth_fun' being undefined
in that case).

src/stun.erl

@@ -584,7 +584,8 @@ prepare_state(Opts, Sock, Peer, SockMod) when is_list(Opts) ->
 	      Opts);
 	_ ->
 	    #state{session_id = ID, sock = Sock, sock_mod = SockMod,
-		   peer = Peer, hook_fun = proplists:get_value(hook_fun, Opts)}
+		   peer = Peer, hook_fun = proplists:get_value(hook_fun, Opts),
+		   auth = anonymous}
     end;
 prepare_state(State, _Sock, Peer, SockMod) ->
     ID = stun_logger:make_id(),