Fix python crash due to missing Py_None reference counting increase

[BsAtHome]

This fixes #3850 by adding reference count increase before returning Py_None.

[andypugh]

I can't even pretend to understand this bug, or the fix. But thanks.

[BsAtHome]

I can't even pretend to understand this bug, or the fix. But thanks.

Quite simple... I introduced the bug when fixing cppcheck errors a long while ago.

Background, every object in python has a reference count. When you get an object, you increase its reference count (informing that the object is in use). When you no longer need it, you decrease the count. Once the count reaches zero, python will automatically discard the object and free the memory. This is true for all objects, including the None object (in C it is called Py_None).

Halmodule had originally return statements using Py_RETURN_NONE, which is a macro that cppcheck is very disagreeable with. These got changed into return Py_None, but that forgot that the object is referenced (in use) one more time at return time and should have had its reference count increased. When the returned None object becomes unused, somewhere else in the code, then the reference count gets decreased again. At some stage, the None object gets discarded because it has experienced more reference decreases than increases, which is, to say it mildly, a bad thing to happen. When it happens, python correctly asserts a programmer too stupid error and crashes the program.