Try all possible mediaselectors even if one succeeds

Simplify and correct

youtube_dl/extractor/bbc.py

@@ -350,6 +350,8 @@ class BBCCoUkIE(InfoExtractor):
 
     def _download_media_selector(self, programme_id):
         last_exception = None
+        formats = []
+        subtitles = None
         for mediaselector_url in self._MEDIASELECTOR_URLS:
             try:
                 formatsAndSubtitles = self._download_media_selector_url(
@@ -359,12 +361,10 @@ class BBCCoUkIE(InfoExtractor):
                     formats += formatsAndSubtitles[0]
                 # subtitles (dict {lang,sttl})
                 if formatsAndSubtitles[1]:
-                    if not subtitles:
-                        subtitles = formatsAndSubtitles[1]
-                    else:
+                    if subtitles:
                         # prioritise the first sttl for each lang
                         formatsAndSubtitles[1].update(subtitles)
-                        subtitles = formatsAndSubtitles[1]    
+                    subtitles = formatsAndSubtitles[1]    
             except BBCCoUkIE.MediaSelectionError as e:
                 if e.id in ('notukerror', 'geolocation', 'selectionunavailable'):
                     last_exception = e