Backport from HEAD:

* modules/ssl/ssl_engine_init.c (ssl_init_proxy_certs): Fail early
(rather than segfault later) if a client cert is configured which is
missing either the certificate or private key.

PR: 24030
Reviewed by: jorton, minfrin, jerenkrantz, wrowe


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/APACHE_2_0_BRANCH@105733 13f79535-47bb-0310-9956-ffa450edef68