pid_namespace: pidns_get() should check task_active_pid_ns() != NULL

pid_namespace: pidns_get() should check task_active_pid_ns() != NULL

pidns_get()->get_pid_ns() can hit ns == NULL. This task_struct can't
go away, but task_active_pid_ns(task) is NULL if release_task(task)
was already called. Alternatively we could change get_pid_ns(ns) to
check ns != NULL, but it seems that other callers are fine.

Signed-off-by: Oleg Nesterov <[email protected]>
Cc: Eric W. Biederman [email protected]>
Cc: [email protected]
Signed-off-by: Linus Torvalds <[email protected]>