crawlers/github: Avoid some potentials nil pointer dereference.
crawlers/github: Avoid some potentials nil pointer dereference.

Refactor formatTimeStamp function to take a pointer as an argument in
order to be able to check for nil pointers.

A nil argument to formatTimeStamp is an error but it should not lead to
a crash, therefore return an unlikely date in this case.

DevMine/crawld