gsocketclient: Fix a leak in the connection code

We miss releasing the async operation's reference on a state object in
one of the error cases.

The call to connection_attempt_remove() (although it calls unref
internally) is not sufficient because this is releasing the reference
that the list owns.

Closes #1774

gio/gsocketclient.c

@@ -1615,6 +1615,7 @@ g_socket_client_connected_callback (GObject      *source,
           set_last_error (data, error);
           connection_attempt_remove (attempt);
           enumerator_next_async (data, FALSE);
+          connection_attempt_unref (attempt);
         }
       else
         {