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 <rsc@swtch.com>