src/libpcp/src/tz.c: add checks for localtime() returning NULL
src/libpcp/src/tz.c: add checks for localtime() returning NULL

In Windows-land, localtime() returns NULL if the input time_t is
- Before midnight, January 1, 1970, or
- After 03:14:07, January 19, 2038, UTC (using _time32 and time32_t), or
- After 23:59:59, December 31, 3000, UTC (using _time64 and __time64_t).

Better to catch this, add a -Dcontext,desperate diagnostic and
return an "epoch" time than take a SEGV.

We were hitting this when compiled on 64-bit Windows where time_t
defaults to a 64-bit value, and we were passing in a pointer to a
32-bit time_t with a bonus 32-bits of junk.

Fixes for the cause in the next commit.

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp