Fix segfault #3604

In async_connect callback, err was clear, but socket->set_option
threw "Bad file descriptor". This caused a segfault.

Side effect of this fix: sometimes a log entry like this shows up. This
happens when it would have faulted.

```
connection failed to localhost:9002: Undefined error: 0
```

plugins/net_plugin/net_plugin.cpp

@@ -1943,7 +1943,7 @@ namespace eosio {
       c->socket->async_connect( current_endpoint, [weak_conn, endpoint_itr, this] ( const boost::system::error_code& err ) {
             auto c = weak_conn.lock();
             if (!c) return;
-            if( !err ) {
+            if( !err && c->socket->is_open() ) {
                start_session( c );
                c->send_handshake ();
             } else {