sync: don't assume b.N > 0

Change-Id: I6eb91ea73ef887b025e5a8de1dd55f30618e1aa6
Reviewed-on: https://go-review.googlesource.com/20857Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/sync/runtime_sema_test.go

@@ -25,6 +25,9 @@ func BenchmarkSemaUncontended(b *testing.B) {
 }
 
 func benchmarkSema(b *testing.B, block, work bool) {
+	if b.N == 0 {
+		return
+	}
 	sem := uint32(0)
 	if block {
 		done := make(chan bool)