ffi: fix segfault in _GET_TZINFO methods

[davidhewitt]

Looks like datetime types without tzinfo set will not have space allocated for the null pointer, so we should check has_tzinfo before trying to read it.

[adamreichold]

The change looks good to me. I have to admit to feeling uneasy about ignoring the different struct layouts between PyDateTime_DateTime and PyDateTime_BaseDateTime. Is it ensured that we never write into a datetime that was allocated by someone else and might be missing that field? Maybe we could add a comment about this to the struct definition?

[pickfire]

Looks good to me, just slightly worried we are not doing the cast like C.

[davidhewitt]

From the datetime docs (https://docs.python.org/3/library/datetime.html#available-types):

Objects of these types are immutable.

I think it's a bug if we ever write to these things.

That said, I still agree we should be careful about the memory layout. I'll push the extra definitions in a follow-up PR.