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/md5module.c

@@ -397,6 +397,7 @@ MD5_hexdigest(MD5object *self, PyObject *unused)
         c = (digest[i] & 0xf);
         hex_digest[j++] = Py_hexdigits[c];
     }
+    assert(_PyUnicode_CheckConsistency(retval, 1));
     return retval;
 }