s3:auth: fix dereference level in talloc checks in create_token_from_sid()

s3:auth: fix dereference level in talloc checks in  create_token_from_sid()

Commit c5b150b33fc54ed97dbd0736cc6f4c15977d6e70 introduced these checks.
The current check "found_username == NULL" is wrong (we would segfault earlier
in this case). We need to check *found_username == NULL instead as
noted by Günter.

Reported-by: Günter Kukkukk <linux@kukkukk.com>
Signed-off-by: Michael Adam <obnox@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>