Fix coding style nits in crypto/asn1/a_strnid.c

Since this file is modified in PR #3934, so should be cleaned up
incidentially.

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4035)