In meos_initialize_timezone, if a timezone is already loaded (session_timezone is not NULL), it is freed. However, the entry that is freed is also the entry that is stored in the cache, so if the new timezone happens to be the same, the new stored timezone is just freed space, that crashes when calling meos_finalize or if changing the timezone again with meos_initialize_timezone.

This PR makes the pg_tzset function to always return a copy of the cached timezone, so it doesn't matter if the returned copy is freed.