SSL: fixed SSL_shutdown() comment.

src/event/ngx_event_openssl.c

diff --git a/src/event/ngx_event_openssl.c b/src/event/ngx_event_openssl.c
index 57dfc6c..1ca1945 100644
--- a/src/event/ngx_event_openssl.c
+++ b/src/event/ngx_event_openssl.c
@@ -1797,7 +1797,7 @@
 
     sslerr = 0;
 
-    /* SSL_shutdown() never returns -1, on error it returns 0 */
+    /* before 0.9.8m SSL_shutdown() returned 0 instead of -1 on errors */
 
     if (n != 1 && ERR_peek_error()) {
         sslerr = SSL_get_error(c->ssl->connection, n);