time: don't match '---' month in time.Parse

The existing implementation will panic when month in date string is '---'.

Fixed #21113

Change-Id: I8058ae7a4102e882f8b7e9c65d80936b563265e4
Reviewed-on: https://go-review.googlesource.com/51010
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/time/format.go

@@ -289,7 +289,6 @@ var shortDayNames = []string{
 }
 
 var shortMonthNames = []string{
-	"---",
 	"Jan",
 	"Feb",
 	"Mar",
@@ -305,7 +304,6 @@ var shortMonthNames = []string{
 }
 
 var longMonthNames = []string{
-	"---",
 	"January",
 	"February",
 	"March",
@@ -841,8 +839,10 @@ func parse(layout, value string, defaultLocation, local *Location) (Time, error)
 			year, err = atoi(p)
 		case stdMonth:
 			month, value, err = lookup(shortMonthNames, value)
+			month++
 		case stdLongMonth:
 			month, value, err = lookup(longMonthNames, value)
+			month++
 		case stdNumMonth, stdZeroMonth:
 			month, value, err = getnum(value, std == stdZeroMonth)
 			if month <= 0 || 12 < month {

src/time/format_test.go

@@ -465,6 +465,9 @@ var parseErrorTests = []ParseErrorTest{
 	{RFC3339, "2006-01-02T15:04:05Z_abc", `parsing time "2006-01-02T15:04:05Z_abc": extra text: _abc`},
 	// invalid second followed by optional fractional seconds
 	{RFC3339, "2010-02-04T21:00:67.012345678-08:00", "second out of range"},
+	// issue 21113
+	{"_2 Jan 06 15:04 MST", "4 --- 00 00:00 GMT", "cannot parse"},
+	{"_2 January 06 15:04 MST", "4 --- 00 00:00 GMT", "cannot parse"},
 }
 
 func TestParseErrors(t *testing.T) {