update_payload: Fix problem with signature size on unsigned payloads

If the payload is unsigned (e.g. for test image), then the checker.sigs_file
would not exist. So check for that variable before using it.

BUG=chromium:794404
BUG=chromium:914705
TEST=manually signing the payload ran update_payload_check with signed and
unsigned image
TEST=unittests

Change-Id: I871137eadef00d012ee926d12fd4eee36a454487
Reviewed-on: https://chromium-review.googlesource.com/1375023


Commit-Ready: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Sen Jiang <senj@chromium.org>