time: fix ParseDuration overflow when given more than 9 digits on 32-bit arch

Fixes #6617.

R=golang-dev, rsc, r
CC=golang-dev
https://golang.org/cl/15080043

src/pkg/time/format.go

@@ -1204,11 +1204,11 @@ func ParseDuration(s string) (Duration, error) {
 			if err != nil {
 				return 0, errors.New("time: invalid duration " + orig)
 			}
-			scale := 1
+			scale := 1.0
 			for n := pl - len(s); n > 0; n-- {
 				scale *= 10
 			}
-			g += float64(x) / float64(scale)
+			g += float64(x) / scale
 			post = pl != len(s)
 		}
 		if !pre && !post {

src/pkg/time/time_test.go

@@ -1318,6 +1318,8 @@ var parseDurationTests = []struct {
 	{"39h9m14.425s", true, 39*Hour + 9*Minute + 14*Second + 425*Millisecond},
 	// large value
 	{"52763797000ns", true, 52763797000 * Nanosecond},
+	// more than 9 digits after decimal point, see http://golang.org/issue/6617
+	{"0.3333333333333333333h", true, 20 * Minute},
 
 	// errors
 	{"", false, 0},