Fix proxy certificate pathlength verification

While travelling up the certificate chain, the internal
proxy_path_length must be updated with the pCPathLengthConstraint
value, or verification will not work properly.  This corresponds to
RFC 3820, 4.1.4 (a).

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Stephen Henson <steve@openssl.org>