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.
diff --git a/src/event/ngx_event_openssl.c b/src/event/ngx_event_openssl.c
index 93ae06a..60d4f61 100644
--- a/src/event/ngx_event_openssl.c
+++ b/src/event/ngx_event_openssl.c
@@ -1232,6 +1232,8 @@
             break;
         }
 
+        flush = 0;
+
         buf->pos = buf->start;
         buf->last = buf->start;