Fix use-after-free in LocalClient

If the DCB was closed before the handshake for the LocalCliet connection
was received, the gw_decode_mysql_server_handshake would use the closed
DCB to log the connection ID. Clearing out the pointer prevents it.

server/modules/protocol/MySQL/mariadb_client.cc

@@ -32,6 +32,8 @@ LocalClient::LocalClient(MYSQL_session* session, MySQLProtocol* proto, int fd)
     , m_self_destruct(false)
 {
     MXB_POLL_DATA::handler = LocalClient::poll_handler;
+    m_protocol.owner_dcb = nullptr;
+    m_protocol.stored_query = nullptr;
 }
 
 LocalClient::~LocalClient()