[tls] Fix potential memory leak

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/tls.c

 	list_del ( &iobuf->list );
 	if ( ! list_empty ( rx_data ) ) {
 		DBGC ( tls, "TLS %p overlength non-data record\n", tls );
+		free_iob ( iobuf );
 		return -EINVAL_NON_DATA;
 	}