avformat/matroskadec: Do not leak queued packets on sync errors

Fixes: memleak
Fixes: clusterfuzz-testcase-minimized-audio_decoder_fuzzer-5649187601121280

Reported-by: Chris Cunningham <chcunningham@google.com>
Tested-by: Chris Cunningham <chcunningham@google.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit d1afa7284c)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/matroskadec.c

@@ -3363,7 +3363,7 @@ static int matroska_read_packet(AVFormatContext *s, AVPacket *pkt)
             ret = matroska_resync(matroska, pos);
     }
 
-    return ret;
+    return 0;
 }
 
 static int matroska_read_seek(AVFormatContext *s, int stream_index,