Fix memory leak in auth-pam plugin

Fix memory leak in auth-pam plugin

As it says on the tin.  aresp would not be free'd nor returned by
my_conv() on errors.  Note that we never reach this code if allocation
of aresp failed.

Found with the Clang static analyzer.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Lev Stipakov <lstipakov@gmail.com>
Message-Id: <1445380706-20864-1-git-send-email-steffan@karger.me>
URL: http://article.gmane.org/gmane.network.openvpn.devel/10338
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit cfc13b38bc6504b9768e4cc43311807d6b074672)