avcodec/decode: decode_simple_internal() only implements audio and video

Fixes: CID1538861 Uninitialized scalar variable

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit e9bb586543)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/decode.c

@@ -449,7 +449,8 @@ FF_ENABLE_DEPRECATION_WARNINGS
     } else if (avctx->codec->type == AVMEDIA_TYPE_AUDIO) {
         ret =  !got_frame ? AVERROR(EAGAIN)
                           : discard_samples(avctx, frame, discarded_samples);
-    }
+    } else
+        av_assert0(0);
 
     if (ret == AVERROR(EAGAIN))
         av_frame_unref(frame);