OCSP stapling: fixed segfault with dynamic certificate loading. If OCSP stapling was enabled with dynamic certificate loading, with some OpenSSL versions (1.0.2o and older, 1.1.0h and older; fixed in 1.0.2p, 1.1.0i, 1.1.1) a segmentation fault might happen. The reason is that during an abbreviated handshake the certificate callback is not called, but the certificate status callback was called (https://github.com/openssl/openssl/issues/1662), leading to NULL being returned from SSL_get_certificate(). Fix is to explicitly check SSL_get_certificate() result.