Otherwise I see output like

 | Dumping path for 1536412160 fd via self 27 [/usr/bin/screen]

where fd is taken from stack and what is worse -- we work
with root privilegues, thus dumping _anything_ from stack
is very bad idea.

With explicitly cleared stack data and FD_DESC_INVALID the
log file will have an entry like

 | Dumping path for -3 fd via self 27 [/usr/bin/screen]
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>