Fix leaked reference to None.

Objects/unicodeobject.c

@@ -2928,6 +2928,7 @@ PyObject *PyUnicode_DecodeCharmap(const char *s,
 		    Py_DECREF(x);
 		    goto onError;
 		}
+		Py_DECREF(x);
 		continue;
 	    }
 	    else if (PyUnicode_Check(x)) {