testing: fix divide by zero when benchmark is faster than OS's clock resolution

I did not added a test because `benchmark_test.go` is `package testing_test`
and I don't care to change that because calling predictN is not testing the
thing I would want to test.

Ideally we would run benchmark in a VM with a highjacked clocksource that never
marches forward, or using faketime but that looks fairly involved for a quickie
fix.

Fixes #70709

Change-Id: I8b4d697aff7cba33da388cb0ae8e2c2b550b9690
Reviewed-on: https://go-review.googlesource.com/c/go/+/633419
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>

src/testing/benchmark.go

@@ -275,6 +275,11 @@ func (b *B) doBench() BenchmarkResult {
 }
 
 func predictN(goalns int64, prevIters int64, prevns int64, last int64) int {
+	if prevns == 0 {
+		// Round up to dodge divide by zero. See https://go.dev/issue/70709.
+		prevns = 1
+	}
+
 	// Order of operations matters.
 	// For very fast benchmarks, prevIters ~= prevns.
 	// If you divide first, you get 0 or 1,
@@ -321,12 +326,7 @@ func (b *B) launch() {
 				// Predict required iterations.
 				goalns := d.Nanoseconds()
 				prevIters := int64(b.N)
-				prevns := b.duration.Nanoseconds()
-				if prevns <= 0 {
-					// Round up, to avoid div by zero.
-					prevns = 1
-				}
-				n = int64(predictN(goalns, prevIters, prevns, last))
+				n = int64(predictN(goalns, prevIters, b.duration.Nanoseconds(), last))
 				b.runN(int(n))
 			}
 		}