time: fix optional fractional seconds range err

time: fix optional fractional seconds range err

The optional fractional seconds overrides any range error
from the second parsing. Instead don't look for optional fractional
seconds if a range error has occured.

Change-Id: I27e0a2432740f6753668bd8833e48b9495bc4036
Reviewed-on: https://go-review.googlesource.com/27590
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>