Fix regression with password protected private keys (polarssl)

Between versions 1.2.7 and 1.2.8, polarssl changed the errors
returned by the X509 parsing functions, which broke the OpenVPN
implementation for password protected private keys in polarssl
builds. This patch fixes that by checking for the new errors in
OpenVPN.

Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <5432E951.6020405@fox-it.com>
Signed-off-by: Gert Doering <gert@greenie.muc.de>