YoutubeDL: only set the ‘formats’ field of the info_dict if it was already set before

It caused a circular reference error, when trying to dump it to json (for example with the test video for myvideo.de or any other video without formats)

youtube_dl/YoutubeDL.py

@@ -688,7 +688,12 @@ class YoutubeDL(object):
                         ext_ord)
             formats = sorted(formats, key=_free_formats_key)
 
-        info_dict['formats'] = formats
+        if formats[0] is not info_dict: 
+            # only set the 'formats' fields if the original info_dict list them
+            # otherwise we end up with a circular reference, the first (and unique)
+            # element in the 'formats' field in info_dict is info_dict itself, 
+            # wich can't be exported to json
+            info_dict['formats'] = formats
         if self.params.get('listformats', None):
             self.list_formats(info_dict)
             return