avcodec/adpcm: use assert for codec id instead or error return

A unsupported codec_id is a internal error and should not happen
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/adpcm.c

@@ -1679,7 +1679,7 @@ static int adpcm_decode_frame(AVCodecContext *avctx, void *data,
         break;
 
     default:
-        return -1;
+        av_assert0(0); // unsupported codec_id should not happen
     }
 
     if (avpkt->size && bytestream2_tell(&gb) == 0) {