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.

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>