Fix a crash when file:change_time/2,3 are called with invalid dates

Calling file:change_time/2,3 with an invalid date tuple
(e.g file:change_time("file.txt", {undefined, undefined}))
will cause file_server_2 to crash.
error_logger will shutdown and the whole VM will stop.
Change behavior to validate the given dates before doing the server call, i.e on the boundaries.
Merge branch 'master' of https://github.com/spawnthink/otp

Conflicts:
	lib/kernel/src/file.erl