[YoutubeDL] Initialize `files_to_delete` (Closes #5797)

youtube_dl/YoutubeDL.py

@@ -1527,6 +1527,7 @@ class YoutubeDL(object):
             pps_chain.extend(ie_info['__postprocessors'])
         pps_chain.extend(self._pps)
         for pp in pps_chain:
+            files_to_delete = []
             try:
                 files_to_delete, info = pp.run(info)
             except PostProcessingError as e: