[downloader/common] Fix possible negative sleep time in slow_down()

youtube_dl/downloader/common.py

@@ -157,7 +157,7 @@ class FileDownloader(object):
             return
         speed = float(byte_counter) / elapsed
         if speed > rate_limit:
-            time.sleep((byte_counter / rate_limit) - elapsed)
+            time.sleep(max((byte_counter / rate_limit) - elapsed, 0))
 
     def temp_name(self, filename):
         """Returns a temporary filename for the given filename."""