Use `size_t` to stop MSVC griping about conversion.

Change-Id: Id2982ba23fc07a5c190ed262c7ee94b91e277364
Reviewed-on: https://code-review.googlesource.com/c/re2/+/60072
Reviewed-by: Perry Lorier <perryl@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>
1 file changed