time: don't panic stringifying the zero Month

Fixes #17720

Change-Id: Ib95c230deef3934db729856c17908f8e5a1e2b7f
Reviewed-on: https://go-review.googlesource.com/33145
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/time/time.go

@@ -114,7 +114,18 @@ var months = [...]string{
 }
 
 // String returns the English name of the month ("January", "February", ...).
-func (m Month) String() string { return months[m-1] }
+func (m Month) String() string {
+	if January <= m && m <= December {
+		return months[m-1]
+	}
+	const prefix = "%!Month("
+	buf := make([]byte, 20+len(prefix)+1)
+	buf[len(buf)-1] = ')'
+	n := fmtInt(buf[:len(buf)-1], uint64(m))
+	n -= len(prefix)
+	copy(buf[n:], prefix)
+	return string(buf[n:])
+}
 
 // A Weekday specifies a day of the week (Sunday = 0, ...).
 type Weekday int

src/time/time_test.go

@@ -1230,3 +1230,10 @@ func TestMarshalBinaryZeroTime(t *testing.T) {
 		t.Errorf("t0=%#v\nt1=%#v\nwant identical structures", t0, t1)
 	}
 }
+
+// Issue 17720: Zero value of time.Month fails to print
+func TestZeroMonthString(t *testing.T) {
+	if got, want := Month(0).String(), "%!Month(0)"; got != want {
+		t.Errorf("zero month = %q; want %q", got, want)
+	}
+}