netops: catch the server not sending a certificate

It's possible for an encrypted connection not have a certificate. In
this case, SSL_get_verify_result() will return OK because no error
happened (as it never even tried to validate anything).

SSL_get_peer_certificate() will return NULL in this case so we need to
catch that. On the upside, the current code would segfault in this
situation instead of letting it through as a valid cert.

src/netops.c

@@ -287,6 +287,10 @@ static int verify_server_cert(gitno_ssl *ssl, const char *host)
 
 
 	cert = SSL_get_peer_certificate(ssl->ssl);
+	if (!cert) {
+		giterr_set(GITERR_SSL, "the server did not provide a certificate");
+		return -1;
+	}
 
 	/* Check the alternative names */
 	alts = X509_get_ext_d2i(cert, NID_subject_alt_name, NULL, NULL);