sessiond: fix: possible unaligned access in packed structure

Use a temporary snapshot_id in cmd_rotate_get_info() to obtain the
return value of cmd_snapshot_add_output() and explicitly assign it to
the reply communication structure. Otherwise, &reply.id may be
unaligned.

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