Mail: discard pipelined commands after SMTP STARTTLS.

The bug had appeared in nginx 1.5.6 (04e43d03e153).

Reported by Chris Boulton.

src/mail/ngx_mail_smtp_handler.c

@@ -777,6 +777,9 @@ ngx_mail_smtp_starttls(ngx_mail_session_t *s, ngx_connection_t *c)
             ngx_str_null(&s->smtp_from);
             ngx_str_null(&s->smtp_to);
 
+            s->buffer->pos = s->buffer->start;
+            s->buffer->last = s->buffer->start;
+
             c->read->handler = ngx_mail_starttls_handler;
             return NGX_OK;
         }