Address some more `-Wsign-compare` warnings.

Fixes #499.

Change-Id: I5405db7dff30ba84387c89a6064902f948b1ee3c
Reviewed-on: https://code-review.googlesource.com/c/re2/+/63370
Reviewed-by: Alex Chernyakhovsky <achernya@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>