Fix case where transition can be None

[jacebrowning]

When calling .subtract(days=days) with the maximum value ("now" - datetime.min), the following exception is raised:

>       if transition.is_ambiguous(sec):
E       AttributeError: 'NoneType' object has no attribute 'is_ambiguous'

[jacebrowning]

@sdispater Please let me know if there is a cleaner way to solve this.

FYI: The InvalidPosixSpec test seems to be failing on master.

[sdispater]

Thanks for your contribution!