Prevent crash when an object is destroyed after calling PythonQt::cleanup()

In certain situations the dealloc callback
`PythonQtInstanceWrapper_dealloc` is called after `PythonQt::cleanup()` has
been run. This can happen when Python destroys objects during `Py_Finalize()`.

This commit adds a check that PythonQt is still initialized in the dealloc callback.

(cherry picked from commit commontk/PythonQt@e4bd663)

Author: msmolens

src/PythonQtInstanceWrapper.cpp

@@ -127,7 +127,9 @@ static void PythonQtInstanceWrapper_deleteObject(PythonQtInstanceWrapper* self,
 
 static void PythonQtInstanceWrapper_dealloc(PythonQtInstanceWrapper* self)
 {
-  PythonQtInstanceWrapper_deleteObject(self);
+  if (PythonQt::self()) {
+    PythonQtInstanceWrapper_deleteObject(self);
+  }
   self->_obj.~QPointer<QObject>();
   Py_TYPE(self)->tp_free((PyObject*)self);
 }