time: fix Parse for empty seconds

The error return value of the seconds field is overwritten
and not checked in the presence of a fractional second.
Perform an explicit check for errors.

Fixes #54569

Change-Id: I1204c8bdcd5a5a09b773d9e44748141ed1e5cb20
Reviewed-on: https://go-review.googlesource.com/c/go/+/425036
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Rob Pike <r@golang.org>

Author: dsnet

src/time/format.go

@@ -1135,6 +1135,9 @@ func parse(layout, value string, defaultLocation, local *Location) (Time, error)
 			}
 		case stdSecond, stdZeroSecond:
 			sec, value, err = getnum(value, std == stdZeroSecond)
+			if err != nil {
+				break
+			}
 			if sec < 0 || 60 <= sec {
 				rangeErrString = "second"
 				break

src/time/format_test.go

@@ -591,6 +591,8 @@ 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 54569
+	{RFC3339, "0000-01-01T00:00:.0+00:00", `parsing time "0000-01-01T00:00:.0+00:00" as "2006-01-02T15:04:05Z07:00": cannot parse ".0+00:00" as "05"`},
 	// 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"},