time: handle int64 overflow in ParseDuration.

Previously, passing a long duration to ParseDuration could result in
random, even negative, values.

LGTM=r
R=golang-codereviews, bradfitz, r
CC=golang-codereviews
https://golang.org/cl/72120043

src/pkg/time/format.go

@@ -1240,5 +1240,8 @@ func ParseDuration(s string) (Duration, error) {
 	if neg {
 		f = -f
 	}
+	if f < float64(-1<<63) || f > float64(1<<63-1) {
+		return 0, errors.New("time: overflow parsing duration")
+	}
 	return Duration(f), nil
 }

src/pkg/time/time_test.go

@@ -842,6 +842,7 @@ var parseDurationTests = []struct {
 	{"-.", false, 0},
 	{".s", false, 0},
 	{"+.s", false, 0},
+	{"3000000h", false, 0}, // overflow
 }
 
 func TestParseDuration(t *testing.T) {