main/musl: backport fix for TZ parsing logic

This fixes usage of TZ value which do not contain a slash character with musl libc. Such as TZ=CET. I doubt that there will be a new musl release before Alpine's 3.15 feature freeze and I would really like this fix to be included in the next release as it fixes real bugs in real software (see !19695 (closed) for example).

See also: https://www.openwall.com/lists/musl/2021/04/25/1