Add casts for some conversions from size_t.
Add casts for some conversions from size_t.

Fixes #89.

Change-Id: If04a45034cce72b802f8090ac5585074bced685f
Reviewed-on: https://code-review.googlesource.com/4660
Reviewed-by: Paul Wankadia <62d4e38bb1380f055ec4b899baa75e52af09c5f7@google.com>

google/re2,google/re2,google/re2,google/re2,google/re2