addDecorators: Check for null object

This prevents potential crashes or undefined behavior
when a null object is passed to the method.

(cherry picked from commit commontk/PythonQt@751ec46)

Author: jcfr

src/PythonQt.cpp

@@ -1560,6 +1560,10 @@ PythonQtClassInfo* PythonQtPrivate::currentClassInfoForClassWrapperCreation()
 
 void PythonQtPrivate::addDecorators(QObject* o, int decoTypes)
 {
+  if (o == nullptr)
+    {
+    return;
+    }
   o->setParent(this);
   int numMethods = o->metaObject()->methodCount();
   for (int i = 0; i < numMethods; i++) {