Fix test warning.

tests/test_router_raw_empty.cpp

@@ -40,7 +40,7 @@ int main (void) {
     zmq_send(dealer, "", 0, 0);
     
     
-    zmq_msg_t ident, empty, echo;
+    zmq_msg_t ident, empty;
     zmq_msg_init(&ident);
     rc = zmq_msg_recv(&ident, router, 0);
     assert(rc >= 0);
@@ -62,4 +62,4 @@ int main (void) {
     close_zero_linger(dealer);
     close_zero_linger(router);
     zmq_ctx_term(ctx);
-}
\ No newline at end of file
+}