The point of to be able to call localtime()/localtime_r() from another
place inside glib (without reimplementing the #ifdef).

- also handles failures from localtime_r(). It is documented that it
  might fail, so detect the failure.

- in case of failures of localtime(), still initialize the GDate.
  Previously, we may error out with a g_critical() assertion before.
  However, now that failures from localtime_r() are also caught, I think
  we should make an effort to initialize the GDate to something. It
  either way it not supposed to happen.