Prevent crashes during and after cleanup (cont'd)

This is a follow-up to HEAD~3 ("Prevent crashes during and after cleanup")

(cherry picked from commit commontk/PythonQt@8aa183b)

Author: jamesobutler

src/PythonQtObjectPtr.cpp

@@ -172,7 +172,7 @@ PythonQtSafeObjectPtr::~PythonQtSafeObjectPtr()
 {
   if (_object) {
     PYTHONQT_GIL_SCOPE
-    Py_DECREF(_object);
+    if (_object && Py_IsInitialized()) Py_DECREF(_object);
   }
 }