s3:smbd: make smbXsrv_client_connection_pass_loop() more robust

Don't leak fds in the error paths.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11898

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Günther Deschner <gd@samba.org>
