[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'.

test/test_YoutubeDL.py

@@ -15,7 +15,7 @@ from youtube_dl import YoutubeDL
 from youtube_dl.compat import compat_str
 from youtube_dl.extractor import YoutubeIE
 from youtube_dl.postprocessor.common import PostProcessor
-from youtube_dl.utils import match_filter_func
+from youtube_dl.utils import ExtractorError, match_filter_func
 
 TEST_URL = 'http://localhost/sample.mp4'
 
@@ -362,6 +362,13 @@ class TestFormatSelection(unittest.TestCase):
         downloaded_ids = [info['format_id'] for info in ydl.downloaded_info_dicts]
         self.assertEqual(downloaded_ids, ['B', 'C', 'D'])
 
+        ydl = YDL({'format': 'best[height<40]'})
+        try:
+            ydl.process_ie_result(info_dict)
+        except ExtractorError:
+            pass
+        self.assertEqual(ydl.downloaded_info_dicts, [])
+
 
 class TestYoutubeDL(unittest.TestCase):
     def test_subtitles(self):

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