Fix the refleak in strftime when converting a %Z with a user defined tzinfo.

Fix the refleak in strftime when converting a %Z with a user defined tzinfo.
I inverted some of the conditionals to reduce indent levels.  Hopefully
this makes it a little easier to read.

This code caused the leak:

    class FixedOffset(datetime.tzinfo):
        def tzname(self, dt): return "UTC"

    datetime.time(12, 47, tzinfo=FixedOffset()).strftime('%Z')

This code is very tricky and I'm not positive it works.  However,
it neither crashes nor leaks.