Prevent DVDRingBuffer waiting in 'safe_read' during a still frame as this can lead to a deadlock with the player thread if the user tries to call up the OSD menu (due to avcodeclock being locked until the still frame is dismissed, which will never happen if the still frame has an indefinite timeout).
(cherry picked from commit 7ef749b83efd4f3da12353575b6731bc140ba9db)