OpenSSL: check EVP_PKEY key types before returning the pkey

OpenSSL: check EVP_PKEY key types before returning the pkey

The internal EVP_PKEY::pkey member is an union thus we need to check for
the real key type before we can return the corresponding RSA, DSA or EC
public key.

Reported-by: Selva Nair <selva.nair@gmail.com>
Signed-off-by: Emmanuel Deloget <logout@free.fr>
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <e8333f0b838670e558a9fe292cea8988484cd77f.1515775195.git.logout@free.fr>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg16202.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit e603afabb845d2552198843a987b5d9b0b7ac404)