sample-plugins: fix ASN1_STRING_to_UTF8 return value checks

sample-plugins: fix ASN1_STRING_to_UTF8 return value checks

As we did in 2d032c7f for the ASN1_STRING_to_UTF8() calls in the core code,
we should also free(buf) if the function returns 0.

[DS: On-the-fly merge conflict fix:
     There was a conflict against the OpenSSL 0.9.6b workaround in v2.4.
     Since we no longer support anything older than OpenSSL 0.9.8 in
     release/2.4, whack that workaround and be more consistent with git
     master those two places]

Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: David Sommerseth <davids@openvpn.net>
Message-Id: <1501238302-16714-1-git-send-email-steffan.karger@fox-it.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg15161.html
Signed-off-by: David Sommerseth <davids@openvpn.net>
(cherry picked from commit c43045ca0590364552fbd060cc65ee1c50a4866a)