cmd/go: avoid crashing on 'go test -n'

Fix a buglet in the go command support for 'go test -n': check for
nil output buffer in action routine.

Fixes #22644

Change-Id: I2566e3bb3d53d0324c4ddd6fec5d30224bf290df
Reviewed-on: https://go-review.googlesource.com/76710
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/go_test.go

@@ -5060,3 +5060,12 @@ func TestGcflagsPatterns(t *testing.T) {
 	tg.grepStderr("compile.* -N .*-p reflect", "did not build reflect with -N flag")
 	tg.grepStderrNot("compile.* -N .*-p fmt", "incorrectly built fmt with -N flag")
 }
+
+// Issue 22644
+func TestGoTestMinusN(t *testing.T) {
+	// Intent here is to verify that 'go test -n' works without crashing.
+	// This reuses flag_test.go, but really any test would do.
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.run("test", "testdata/flag_test.go", "-n", "-args", "-v=7")
+}

src/cmd/go/internal/test/test.go

@@ -1472,8 +1472,10 @@ func builderCleanTest(b *work.Builder, a *work.Action) error {
 func builderPrintTest(b *work.Builder, a *work.Action) error {
 	clean := a.Deps[0]
 	run := clean.Deps[0]
-	os.Stdout.Write(run.TestOutput.Bytes())
-	run.TestOutput = nil
+	if run.TestOutput != nil {
+		os.Stdout.Write(run.TestOutput.Bytes())
+		run.TestOutput = nil
+	}
 	return nil
 }