metrics: fix global-buffer-overflow

This fix the problem that the subscript of a global
array overflows in some cases.

BUG=chromium:326284
TEST=Unittest and suite:smoke passes.

Change-Id: I6a19d3bb18a886ee8a29b9a4df5f2b136ce57c5c
Reviewed-on: https://chromium-review.googlesource.com/178973


Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Yunlian Jiang <yunlian@chromium.org>
Tested-by: Yunlian Jiang <yunlian@chromium.org>