SSL: resetting of flush flag after the data was written.

There is no need to flush next chunk of data if it does not contain a buffer
with the flush or last_buf flags set.

src/event/ngx_event_openssl.c

@@ -1232,6 +1232,8 @@ ngx_ssl_send_chain(ngx_connection_t *c, ngx_chain_t *in, off_t limit)
             break;
         }
 
+        flush = 0;
+
         buf->pos = buf->start;
         buf->last = buf->start;