[dropbox] Fix the video url query (fixes #3868)

Previously it would convert urls without a query into 'something.mp4&dl=1'

youtube_dl/extractor/dropbox.py

@@ -29,9 +29,8 @@ class DropboxIE(InfoExtractor):
         video_id = mobj.group('id')
         fn = compat_urllib_parse_unquote(url_basename(url))
         title = os.path.splitext(fn)[0]
-        video_url = (
-            re.sub(r'[?&]dl=0', '', url) +
-            ('?' if '?' in url else '&') + 'dl=1')
+        video_url = re.sub(r'[?&]dl=0', '', url)
+        video_url += ('?' if '?' not in video_url else '&') + 'dl=1'
 
         return {
             'id': video_id,