Fix SSL_CTX_get{first,next}_certificate.

Fix typo that stopped SSL_CTX_get_{first,next}_certificate from working.
diff --git a/ssl/ssl_cert.c b/ssl/ssl_cert.c
index 385d25f..4e75a96 100644
--- a/ssl/ssl_cert.c
+++ b/ssl/ssl_cert.c
@@ -670,7 +670,7 @@
 		return 0;
 	for (i = idx; i < SSL_PKEY_NUM; i++)
 		{
-		CERT_PKEY *cpk = c->key + i;
+		CERT_PKEY *cpk = c->pkeys + i;
 		if (cpk->x509 && cpk->privatekey)
 			{
 			c->key = cpk;