localtime can return NULL if unable to read system clock

localtime can return NULL if unable to read system clock

This adds checks for some invocations of localtime() to avoid possible
NULL dereference. (via facebook-infer)

Change-Id: I2b779d8f60c032563eb4ee3cebe20b14afbb0fa3
Reviewed-on: https://gerrit.openafs.org/13206
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>