Fix the newly-introduced warning and turn on -Werror.

I cleaned up most of our warnings last week but forgot to turn on -Werror,
so of course we're getting new warnings already. I've left -Werror commented
out in those places where we still have warnings to deal with before we can
turn on -Werror.

Change-Id: Ia58ff8b8c1ada4bf81eec6f19ec1d34e133cf4b1