[YoutubeDL] format spec: Do not fail when a filter gives an empty result

For example with 'best[height<40]' we ended getting a 'IndexError: list index out of range'.

1 files changed, 3 insertions, 0 deletions

diff --git a/youtube_dl/YoutubeDL.py b/youtube_dl/YoutubeDL.py
index 5a79e5f1d..6478d05dc 100755
--- a/youtube_dl/YoutubeDL.py
+++ b/youtube_dl/YoutubeDL.py
@@ -1005,6 +1005,9 @@ class YoutubeDL(object):
                 format_spec = selector.selector
 
                 def selector_function(formats):
+                    formats = list(formats)
+                    if not formats:
+                        return
                     if format_spec == 'all':
                         for f in formats:
                             yield f