fix 64 to 32 bit clang conversion warning

Change-Id: I62e86c0e19800e128c2d2fb5f952c9ee7a7b4258
Reviewed-on: https://code-review.googlesource.com/c/re2/+/60210
Reviewed-by: Perry Lorier <perryl@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>
1 file changed