vcl: hold errno when calling LDBG

vcl: hold errno when calling LDBG

Type: fix

Call trace of LDBG:
LDBG->clib_warning->_clib_error->dispatch_message->os_puts->writev
However, writev will hijacked by LDP, and then execute following code:
if ((errno = -ldp_init ()))
    return -1;
Now, errno will be set.

Because we always call LDBG just before return from ldp_accept4, listen,
and etc. So errno will be overwritted after LDBG called.

Signed-off-by: hanlin <hanlin_wang@163.com>
Change-Id: I7a90f3a14772994f11f09650481411796e3f5630