runtime: don't assume b.N > 0

Change-Id: I2e26717f2563d7633ffd15f4adf63c3d0ee3403f
Reviewed-on: https://go-review.googlesource.com/20856
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/proc_test.go

@@ -428,6 +428,9 @@ func TestPingPongHog(t *testing.T) {
 }
 
 func BenchmarkPingPongHog(b *testing.B) {
+	if b.N == 0 {
+		return
+	}
 	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(1))
 
 	// Create a CPU hog