Address another warning from MSVC.
Address another warning from MSVC.

For #142.

Change-Id: I0379209185208ffcd7125292ca662878e48e37aa
Reviewed-on: https://code-review.googlesource.com/12710
Reviewed-by: Paul Wankadia <62d4e38bb1380f055ec4b899baa75e52af09c5f7@google.com>

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