Fix sign-compare warnings

This fixes the following warning:

warning: comparison between signed and unsigned integer expressions
[-Wsign-compare]

Tested on Linux with the following command line:

$ bazel test :all

Change-Id: Ie2eefaf94e9bf7063a2bb01dfad13131eb0673e7
Reviewed-on: https://code-review.googlesource.com/3781
Reviewed-by: Paul Wankadia <junyer@google.com>
2 files changed