Fix `--skip-unavailable-fragments`

Bug in d71fd412
Closes #3437

yt_dlp/downloader/fragment.py

@@ -134,6 +134,8 @@ def _download_fragment(self, ctx, frag_url, info_dict, headers=None, request_dat
         return True
 
     def _read_fragment(self, ctx):
+        if not ctx.get('fragment_filename_sanitized'):
+            return None
         try:
             down, frag_sanitized = self.sanitize_open(ctx['fragment_filename_sanitized'], 'rb')
         except FileNotFoundError: