turn: do not destroy turn if already destroying

Change-Id: I96ab7002326dd22d064090219f35cabbeb4c2567

src/turn_transport.cpp

@@ -186,7 +186,7 @@ public:
 
 TurnTransportPimpl::~TurnTransportPimpl()
 {
-    if (relay) {
+    if (relay && state.load() != RelayState::DOWN) {
         try {
             pj_turn_sock_destroy(relay);
         } catch (...) {