Only consider sockets that are in valid states as identd candidates

When an identd query happens, we check all the connected sockets for their
local and remote ports, to compare them with the query.  But we MUST only
consider the connected ones because:

- It doesn’t make sense to answer for a connection that has just been closed
- A non-connected sockets is not in a valid state for this (specifically
  here: its ::port attribute is empty)

fix #3421

src/network/tcp_client_socket_handler.cpp

@@ -263,6 +263,8 @@ std::string TCPClientSocketHandler::get_port() const
 
 bool TCPClientSocketHandler::match_port_pair(const uint16_t local, const uint16_t remote) const
 {
+  if (!this->is_connected())
+    return false;
   const auto remote_port = static_cast<uint16_t>(std::stoi(this->port));
-  return this->is_connected() && local == this->local_port && remote == remote_port;
+  return local == this->local_port && remote == remote_port;
 }