poll.c:dcb_close Don't call poll_remove_dcb anymore if DCB has already been removed from poll set.

mysql_backend.c, mysql_client.c free error message GWBUF after calling handleError
readconnroute.c:handleError send error message to client before returning.
readwritesplit.c:handleError don't free error message buffer anymore since the caller of handleError frees it.

server/modules/protocol/mysql_client.c

@@ -859,6 +859,7 @@ int gw_read_client_event(
                                                     dcb,
                                                     ERRACT_REPLY_CLIENT,
                                                     &succp);
+				gwbuf_free(errbuf);
                                 ss_dassert(!succp);
 
                                 dcb_close(dcb);