[utils] Do not fail on unknown date formats in unified_strdate

youtube_dl/utils.py

@@ -1055,7 +1055,10 @@ def unified_strdate(date_str, day_first=True):
     if upload_date is None:
         timetuple = email.utils.parsedate_tz(date_str)
         if timetuple:
+            try:
                 upload_date = datetime.datetime(*timetuple[:6]).strftime('%Y%m%d')
+            except ValueError:
+                pass
     if upload_date is not None:
         return compat_str(upload_date)