CVE-2019-10197: smbd: make sure that change_to_user_internal() always resets current_...

CVE-2019-10197: smbd: make sure that change_to_user_internal() always resets current_user.done_chdir

We should not leave current_user.done_chdir as true if we didn't call
chdir_current_service() with success.

This caused problems in when calling vfs_ChDir() in pop_conn_ctx() when
chdir_current_service() worked once on one share but later failed on another
share.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14035

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>