Fix: sessiond: Dereference before null check

Fix: sessiond: Dereference before null check

Coverity report:
  CID 1404933 (#1 of 1): Dereference before null check
  (REVERSE_INULL)check_after_deref: Null-checking
  session->chunk_being_archived suggests that it may be null, but it has
  already been dereferenced on all paths leading to the check.

Reported-by: Coverity (1404933) Dereference before null check
Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>