s is a uint32 and can never be zero. It's max value is already tested
against sig.wanted, whose size is derived from _NSIG.  This also
matches the test in signal_enable.

Fixes #11282

Change-Id: I8eec9c7df8eb8682433616462fe51b264c092475
Reviewed-on: https://go-review.googlesource.com/13940Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>