Update MP4Recorder.cpp

fix #1679 
_muxer->openMP4 创建非法的文件夹会失败抛出异常  所以_full_path_tmp等都是空字符串
delete_file的时候 就会从此盘符的""(full_path_tmp)目录挨个删除了。 
复现建议新建个盘符...

src/Record/MP4Recorder.cpp

@@ -73,6 +73,8 @@ void MP4Recorder::asyncClose() {
         info.time_len = (float) (::time(NULL) - info.start_time);
         //关闭mp4非常耗时，所以要放在后台线程执行
         muxer->closeMP4();
+        
+        if(!full_path_tmp.empty()) {
             //获取文件大小
             info.file_size = File::fileSize(full_path_tmp.data());
             if (info.file_size < 1024) {
@@ -82,6 +84,7 @@ void MP4Recorder::asyncClose() {
             }
             //临时文件名改成正式文件名，防止mp4未完成时被访问
             rename(full_path_tmp.data(), full_path.data());
+        }
 
         /////record 业务逻辑//////
         NoticeCenter::Instance().emitEvent(Broadcast::kBroadcastRecordMP4, info);