Address some `-Wsign-compare` warnings.

Fixes #499.

Change-Id: I076d4fb4a51e0589176fdd175045ac90b0e361cb
Reviewed-on: https://code-review.googlesource.com/c/re2/+/63350
Reviewed-by: Paul Wankadia <junyer@google.com>
Reviewed-by: Alex Chernyakhovsky <achernya@google.com>