Fix X509_NAME decode for malloc failures.

The original X509_NAME decode free code was buggy: this
could result in double free or leaks if a malloc failure
occurred.

Simplify and fix the logic.

Thanks to Guido Vranken for reporting this issue.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1691)
(cherry picked from commit 6dcba070)