Fix: Required to lock TCPIP core functionality

See: espressif/arduino-esp32#10526 (comment)

src/AsyncTCP.cpp

@@ -1531,10 +1531,10 @@ void AsyncServer::end(){
         TCP_MUTEX_LOCK();
         tcp_arg(_pcb, NULL);
         tcp_accept(_pcb, NULL);
-        TCP_MUTEX_UNLOCK();
         if(tcp_close(_pcb) != ERR_OK){
             _tcp_abort(_pcb, -1);
         }
+        TCP_MUTEX_UNLOCK();
         _pcb = NULL;
     }
 }