Fix another sign-compare warning.
Fix another sign-compare warning.

Change-Id: Ie2dfcb88dc2e0bcff1a3c342105973b5eab8c17c
Reviewed-on: https://code-review.googlesource.com/3790
Reviewed-by: Paul Wankadia <62d4e38bb1380f055ec4b899baa75e52af09c5f7@google.com>

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