type_call() now detect bugs in type new and init

* Call _Py_CheckFunctionResult() to check for bugs in type
  constructors (tp_new)
* Add assertions to ensure an exception was raised if tp_init failed
  or that no exception was raised if tp_init succeed

Refactor also the function to have less indentation.

Objects/typeobject.c

@@ -906,7 +906,10 @@ type_call(PyTypeObject *type, PyObject *args, PyObject *kwds)
 #endif
 
     obj = type->tp_new(type, args, kwds);
-    if (obj != NULL) {
+    obj = _Py_CheckFunctionResult((PyObject*)type, obj, NULL);
+    if (obj == NULL)
+        return NULL;
+
     /* Ugly exception: when the call was type(something),
        don't call tp_init on the result. */
     if (type == &PyType_Type &&
@@ -914,17 +917,22 @@ type_call(PyTypeObject *type, PyObject *args, PyObject *kwds)
         (kwds == NULL ||
          (PyDict_Check(kwds) && PyDict_Size(kwds) == 0)))
         return obj;
+
     /* If the returned object is not an instance of type,
        it won't be initialized. */
     if (!PyType_IsSubtype(Py_TYPE(obj), type))
         return obj;
+
     type = Py_TYPE(obj);
     if (type->tp_init != NULL) {
         int res = type->tp_init(obj, args, kwds);
         if (res < 0) {
+            assert(PyErr_Occurred());
             Py_DECREF(obj);
             obj = NULL;
         }
+        else {
+            assert(!PyErr_Occurred());
         }
     }
     return obj;