Correct redundancy handling with lost/DTX frames

In https://github.com/xiph/opus/issues/253, the encoder generates a Hybrid frame with redundancy, to switch to CELT-only mode, and then activates DTX immediately afterwards. The decoder ran Hybrid PLC, which isn't right. Use CELT PLC instead if there was already a transition to CELT via redundancy at the end of the previous frame.

Also do not use a stale CELT decoder to decode a second redundancy frame when the first redundancy frame for a transition from SILK-only mode was lost. Instead of mixing in old audio from the last time that CELT was used, ignore the second redundancy frame in this case. Alternatively the CELT decoder could be reset before decoding, but it would not be ready until after the 2.5 ms of audio that is needed.