libavformat/matroskadec.c:718: error: Uninitialized Value
  The value read from length was never initialized.

libavformat/matroskadec.c:718:28: 
716. 
717.     switch (syntax->type) {
718.     case EBML_UINT:  res = ebml_read_uint  (pb, length, data);  break;
                                ^
719.     case EBML_FLOAT: res = ebml_read_float (pb, length, data);  break;
720.     case EBML_STR:
