asn1/tasn_utl.c: fix logical error in asn1_do_lock.

CRYPTO_atomic_add was assumed to return negative value on error, while
it returns 0.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/6843)