Segfault when emitting a signal with a None parameter from another thread

(Note this was: http://bugs.pyside.org/show_bug.cgi?id=1152)

See the attached example for a demonstration of the problem.
I'll also attach the traceback I got from gdb.

Essentially what appears to happen is this:

1. PySide::MetaFunction::call converts the arguments to the signal to cpp
values
2. The shiboken converter to do that converts Py_None to a NULL-pointer
(pythonToCppPointer in libshiboken/sbkconvert.cpp)
3. The QMetaObject system tries to make a copy of the PyObject pointer using a
PySide::PyObjectWrapper which then goes boom when trying to INCREF its
underlying python object

I have attached a simple patch that uses Py_XINCREF/DECREF instead. With
this I no longer get a crash, but I don't know the pyside internals well
enough to know if this is a good solution or just masking the symptoms of the
real problem.

Versions used:

apiextractor 0.10.10
generatorrunner 0.6.16
shiboken 1.1.0
pyside 1.1.0

Additional notes:
I verified that the bug is still present in the current gittorious/gerrit repositories

The original bug also contained this comment:

> Hugo Parente Lima 2012-02-27 19:06:49 CET
> Maybe would be nice to avoid create the PyObjectWrapper instance with a null pointer instead of using X_INC/DECREF's