Fix a null-pointer dereference in establish_http_proxy_passthru()

Fix a null-pointer dereference in establish_http_proxy_passthru()

Prevents that the client crashes if the peer does not specify
the 'realm' and/or 'nonce' values. These pointers are
dereferenced in DigestCalcHA1() and DigestCalcResponse();
hence, if not set, a null-pointer dereference would occur.

Signed-off-by: Guido Vranken <guidovranken@gmail.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1497574736-2092-1-git-send-email-gv@guidovranken.nl>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg14844.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>