time: fix Parse for empty seconds

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>