If the netns image is absent, the NetnsEntry entry will not be initialized.

Currently restore from old images crashes:

Core was generated by `criu swrk 3'.
Program terminated with signal SIGSEGV, Segmentation fault.
$0  0x0000000000427d80 in netns_entry.free_unpacked ()
(gdb) bt
$0  0x0000000000427d80 in netns_entry.free_unpacked ()
$1  0x0000000000436d07 in prepare_net_ns ()
$2  0x0000000000457c78 in prepare_namespace ()
$3  0x0000000000432917 in restore_task_with_children ()
$4  0x00007fc86acfccfd in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:111

v2: remove debugging code

Cc: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Acked-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>