lsns: fix netns use

lsns: fix netns use

 # ip netns add vpn
 # lsns -T -t net
 Segmentation fault (core dumped)

The function interpolate_missing_namespaces() reads data from /proc.
However, in the case of a persistent namespace, there is no procfs
entry for the namespace. Therefore, this function should ignore it.

Fixes: https://github.com/util-linux/util-linux/issues/2982
Signed-off-by: Karel Zak <kzak@redhat.com>