Check newly created consistency using _PyUnicode_CheckConsistency(str, 1)

* In debug mode, fill the string data with invalid characters
 * Simplify also reference counting in PyCodec_BackslashReplaceErrors()
   and PyCodec_XMLCharRefReplaceError()

Python/compile.c

@@ -263,6 +263,7 @@ _Py_Mangle(PyObject *privateobj, PyObject *ident)
         Py_DECREF(result);
         return NULL;
     }
+    assert(_PyUnicode_CheckConsistency(result, 1));
     return result;
 }