Fix a use-after-free issue with TimeZone

Summary: I'm trying to debug a double free issue with
TimeZone. I don't think this is it, but in the case
of a race, GetTimeZoneInfoRaw would have returned
a raw pointer, right after freeing it. So there was
certainly a use-after-free issue here.

Reviewed By: @ottoni

Differential Revision: D1651248