metrics: fix buffer underrun when reading empty consent id

BUG=chromium:681925
TEST=running metrics tests w/asan doesn't fail anymore

Change-Id: If51128ebef9ae3d6d1848fbb02389fc5bc38e4d0
Reviewed-on: https://chromium-review.googlesource.com/428951
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Rahul Chaudhry <rahulchaudhry@chromium.org>
1 file changed