avoid crash in case of plain http

[Robert]

diff --git a/src/event/ngx_event_openssl.c b/src/event/ngx_event_openssl.c
index 0527c9c..822d7ba 100644
--- a/src/event/ngx_event_openssl.c
+++ b/src/event/ngx_event_openssl.c
@@ -2198,8 +2198,7 @@ ngx_ssl_get_serial_number(ngx_connection_t *c,
ngx_pool_t *pool, ngx_str_t *s)

     s->len = 0;

-    cert = SSL_get_peer_certificate(c->ssl->connection);
-    if (cert == NULL) {
+    if ( NULL == c->ssl || NULL == ( cert =
SSL_get_peer_certificate(c->ssl->connection))) {
         return NGX_OK;
     }