例程中上传文件如果为空，会引发内存错误

[lazyguoguo]

复现步骤：7. Upload File Test 中 未选择任何文件直接点上传。
日志如下：

Upload FileName: 
Content-Type   : application/octet-stream
save to: /webnet/upload/
((((rt_ubase_t)rmem) & (RT_ALIGN_SIZE - 1)) == 0) assertion failed at function:rt_free, line number:555