Decoder crashes when encoding with command options: --MMVD=0 --FastMrg=0 --MaxNumMergeCand=1 --MaxNumGeoCand=0

[tsukuba.takeshi]

At VTM-8.0 or the latest VTM, decoder crashes when encoding with command options: --MMVD=0 --FastMrg=0 --MaxNumMergeCand=1 --MaxNumGeoCand=0.

This bug comes from the candNum setting at void EncCu::xCheckRDCostMerge2Nx2N(...).

When using option: --MMVD=0 --FastMrg=0 --MaxNumMergeCand=1 --MaxNumGeoCand=0,
the parameter candNum is still set to MRG_MAX_NUM_CANDS.
Thus, RdModeList can contain MMVD candidate at for-loop although MMVD is disabled.

EncCu::xCheckRDCostMerge2Nx2N(...){
...
  const int candNum = MRG_MAX_NUM_CANDS + (tempCS->sps->getUseMMVD() ? MMVD_ADD_NUM : 0);

  for (int i = 0; i < candNum; i++)
  {
    if (i < mergeCtx.numValidMergeCand)
    {
      RdModeList.push_back(ModeInfo(i, true, false, false));
    }
    else
    {
      RdModeList.push_back(ModeInfo(std::min(MMVD_ADD_NUM, i - mergeCtx.numValidMergeCand), false, true, false));
    }
  }
...
}

candNum should be set to mergeCtx.numValidMergeCand + (tempCS->sps->getUseMMVD() ? MMVD_ADD_NUM : 0).

[tsukuba.takeshi]

Suggested fix is ​https://vcgit.hhi.fraunhofer.de/jvet/VVCSoftware_VTM/-/merge_requests/1591.