Strip trailing slash from base

Co-authored-by: pukkandan <pukkandan.ytdlp@gmail.com>

yt_dlp/extractor/common.py

@@ -2334,7 +2334,7 @@ def _parse_smil_formats_and_subtitles(
         for meta in smil.findall(self._xpath_ns('./head/meta', namespace)):
             b = meta.get('base') or meta.get('httpBase')
             if b:
-                base = f'{b}/'
+                base = f'{b.rstrip("/")}/'
                 break
 
         formats, subtitles = [], {}