[downloader/fragment] Remove assert for resume_len when no fragments downloaded

This may be incorrect due some header (e.g. flv header in f4m downloader)

youtube_dl/downloader/fragment.py

@@ -155,8 +155,6 @@ class FragmentFD(FileDownloader):
                 self._write_ytdl_file(ctx)
             if ctx['fragment_index'] > 0:
                 assert resume_len > 0
-            else:
-                assert resume_len == 0
 
         dest_stream, tmpfilename = sanitize_open(tmpfilename, open_mode)