OpenSSL: Use library wrapper functions to access cert store

OpenSSL: Use library wrapper functions to access cert store

OpenSSL 0.9.8 and newer includes SSL_CTX_get_cert_store() and
SSL_CTX_set_cert_store() helper functions, so there is no need to
dereference the SSL_CTX pointer to cert ssl_ctx->cert_store. This helps
in working with the future OpenSSL 1.1.0 release that makes the SSL_CTX
structure opaque.

Signed-off-by: Jouni Malinen <j@w1.fi>