fix a -Wsign-compare warning https://code-review.googlesource.com/#/c/1272/ turned on -Wsign-compare, but missed one place where the warning fires. Change-Id: I0739d935b96da093a7151784274fdb7335e7f5cc Reviewed-on: https://code-review.googlesource.com/1550 Reviewed-by: Russ Cox <[email protected]>