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()

Modules/_json.c

@@ -246,6 +246,7 @@ ascii_escape_unicode(PyObject *pystr)
         }
     }
     output[chars++] = '"';
+    assert(_PyUnicode_CheckConsistency(rval, 1));
     return rval;
 }