[release-branch.go1.15] time: fix LoadLocationFromTZData with slim tzdata

The extend information of a time zone file with last transition < now
could result in a wrong cached zone because it used the zone of the
last transition.

This could lead to wrong zones in systems with slim zoneinfo.

For #42216
Fixes #42138

Change-Id: I7c57c35b5cfa58482ac7925b5d86618c52f5444d
Reviewed-on: https://go-review.googlesource.com/c/go/+/264939


Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
(cherry picked from commit 70e022e4)
Reviewed-on: https://go-review.googlesource.com/c/go/+/266299


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>