Fix bug in SignalHandler (#27)

actors.go

@@ -48,7 +48,8 @@ func SignalHandler(ctx context.Context, signals ...os.Signal) (execute func() er
 type testSigChanKey struct{}
 
 func getTestSigChan(ctx context.Context) <-chan os.Signal {
-	return ctx.Value(testSigChanKey{}).(<-chan os.Signal) // can be nil
+	c, _ := ctx.Value(testSigChanKey{}).(<-chan os.Signal) // can be nil
+	return c
 }
 
 func putTestSigChan(ctx context.Context, c <-chan os.Signal) context.Context {

actors_test.go

@@ -57,3 +57,11 @@ func TestSignalError(t *testing.T) {
 		t.Errorf("errors.Is(err, ErrSignal): want %v, have %v", want, have)
 	}
 }
+
+func TestSignalHandlerNil(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	var rg Group
+	rg.Add(SignalHandler(ctx, os.Interrupt))
+	cancel()
+	t.Logf("%v", rg.Run())
+}