Fix OpenSSL private key passphrase notices

Fix OpenSSL private key passphrase notices

Clear error stack on successful certificate loading in
tls_ctx_load_cert_file_and_copy() and handle errors also for
PEM_read_bio_PrivateKey() call in tls_ctx_load_priv_file().

Due to certificate loading possibly leaking non-fatal errors on OpenSSL
error stack, and some slight oversights in error handling, the

>PASSWORD:Verification Failed: 'Private Key'

line was never produced on the management channel for PEM formatted keys.

Signed-off-by: Santtu Lakkala <santtu.lakkala@jolla.com>
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <20191021113506.30377-1-santtu.lakkala@jolla.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg18953.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit f67efa9412a62f477aa17c3179b7e9f31ac4b25f)