[iOS] Fix md5 potential crash. (#1871)
[iOS] Fix md5 potential crash. (#1871)


acton393/incubator-weex,alibaba/weex,acton393/incubator-weex,alibaba/weex,KalicyZhou/incubator-weex,acton393/incubator-weex,KalicyZhou/incubator-weex,alibaba/weex,KalicyZhou/incubator-weex,acton393/incubator-weex,acton393/incubator-weex,alibaba/weex,KalicyZhou/incubator-weex,acton393/incubator-weex,alibaba/weex,alibaba/weex,KalicyZhou/incubator-weex,KalicyZhou/incubator-weex,alibaba/weex,acton393/incubator-weex,acton393/incubator-weex