libavformat/matroskadec.c:802: error: Uninitialized Value
  The value read from length was never initialized.

libavformat/matroskadec.c:802:29: 
800.     case EBML_PASS:  return ebml_parse_id(matroska, syntax->def.n, id, data);
801.     case EBML_STOP:  return 1;
802.     default:         return url_fseek(pb,length,SEEK_CUR)<0 ? AVERROR(EIO) : 0;
                                 ^
803.     }
804.     if (res == AVERROR_INVALIDDATA)
