[mpc8] Check return value of avio_seek and avoid modifying state if it fails

libavformat/mpc8.c

@@ -274,7 +274,8 @@ static int mpc8_read_seek(AVFormatContext *s, int stream_index, int64_t timestam
     int index = av_index_search_timestamp(st, timestamp, flags);
 
     if(index < 0) return -1;
-    avio_seek(s->pb, st->index_entries[index].pos, SEEK_SET);
+    if (avio_seek(s->pb, st->index_entries[index].pos, SEEK_SET) < 0)
+        return -1;
     c->frame = st->index_entries[index].timestamp;
     return 0;
 }