time: panic with a more helpful error on use of invalid Timer

Fixes #8721

LGTM=rsc
R=r, rsc
CC=golang-codereviews
https://golang.org/cl/155620045

src/time/sleep.go

@@ -55,6 +55,9 @@ type Timer struct {
 // Stop does not close the channel, to prevent a read from the channel succeeding
 // incorrectly.
 func (t *Timer) Stop() bool {
+	if t.r.f == nil {
+		panic("time: Stop called on uninitialized Timer")
+	}
 	return stopTimer(&t.r)
 }
 
@@ -78,6 +81,9 @@ func NewTimer(d Duration) *Timer {
 // It returns true if the timer had been active, false if the timer had
 // expired or been stopped.
 func (t *Timer) Reset(d Duration) bool {
+	if t.r.f == nil {
+		panic("time: Reset called on uninitialized Timer")
+	}
 	w := when(d)
 	active := stopTimer(&t.r)
 	t.r.when = w

src/time/sleep_test.go

@@ -9,6 +9,7 @@ import (
 	"fmt"
 	"runtime"
 	"sort"
+	"strings"
 	"sync"
 	"sync/atomic"
 	"testing"
@@ -407,3 +408,23 @@ func TestOverflowRuntimeTimer(t *testing.T) {
 	// the end of CheckRuntimeTimerOverflow in internal_test.go.
 	CheckRuntimeTimerOverflow()
 }
+
+func checkZeroPanicString(t *testing.T) {
+	e := recover()
+	s, _ := e.(string)
+	if want := "called on uninitialized Timer"; !strings.Contains(s, want) {
+		t.Errorf("panic = %v; want substring %q", e, want)
+	}
+}
+
+func TestZeroTimerResetPanics(t *testing.T) {
+	defer checkZeroPanicString(t)
+	var tr Timer
+	tr.Reset(1)
+}
+
+func TestZeroTimerStopPanics(t *testing.T) {
+	defer checkZeroPanicString(t)
+	var tr Timer
+	tr.Stop()
+}