Fix: session_find_by_id can return NULL legitimately

session_find_by_id can return NULL when the backing hash table
is NULL. This is not an error in the context of
its caller, save_per_pid_lost_discarded_counters(), since the
ltt_session can be destroyed before an ust app session is torn down.

See the comment in save_per_pid_lost_discarded_counters() for more
information.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>