Use consistent variable names

In the X509_NAME_get_index_by_NID.pod example, the initialized variable is called
"loc", but the one used in the for loop is called "lastpos". Make the names match.

CLA: trivial
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1949)