Fix broken loading of client CAs

The SSL_load_client_CA_file() failed to load any CAs due to an
inccorrect assumption about the return value of lh_*_insert(). The
return value when inserting into a hash is the old value of the key.

The bug was introduced in 3c82e437bb3af822ea13cd5a24bab0745c556246.

Reviewed-by: Kurt Roeckx <kurt@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1279)