libavformat/matroskadec.c:719: error: Uninitialized Value
  The value read from length was never initialized.

libavformat/matroskadec.c:719:28: 
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:
721.     case EBML_UTF8:  res = ebml_read_ascii (pb, length, data);  break;
