Guard against null t->tm_zone in strftime.c.

 Guard against null t->tm_zone in strftime.c.
 
 The upstream IANA code does not guard against null TM_ZONE pointers in this
 function, but in our code there is such a check in the other pre-existing
 use of t->tm_zone.  We do have some places that set pg_tm.tm_zone to NULL.
 I'm not entirely sure it's possible to reach strftime with such a value,
 but I'm not sure it isn't either, so be safe.
 
 Per Coverity complaint.