Do not check DaylightName if DynamicDaylightTimeDisabled is false.

Fixes #13502

Change-Id: I03e01ec0afdeb0037a5d3ee243674e5a6b559da5
Reviewed-on: https://go-review.googlesource.com/17998Reviewed-by: Yasuhiro MATSUMOTO <mattn.jp@gmail.com>
Reviewed-by: Russ Cox <rsc@golang.org>