Constify s2i_ASN1_IA5STRING

Return directly NULL after ASN1_STRING_set, as it already has set an error code.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Kurt Roeckx <kurt@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1074)