LwipClient - op bool fix

op bool should return false only for a 'null' client. a closed client
may still have data to read.
a Client returned from Server.available() should not fail on usual bool
test if it has data available.

Author: JAndrassy

Diff for: libraries/lwIpWrapper/src/lwipClient.cpp

@@ -243,7 +243,7 @@ uint8_t lwipClient::status()
 lwipClient::operator bool()
 {
     /* -------------------------------------------------------------------------- */
-    return (_tcp_client && (_tcp_client->state != TCP_CLOSING));
+    return (_tcp_client != nullptr);
 }
 
 /* -------------------------------------------------------------------------- */