PVS-Studio warning V512 for Doom3

void idAASFileLocal::DeleteClusters( void ) {
  ...
  memset( &portal, 0, sizeof( portal ) );
  portals.Append( portal );

  // first cluster is a dummy
  memset( &cluster, 0, sizeof( portal ) );
  clusters.Append( cluster );
}

This is what should have been written here:
memset( &cluster, 0, sizeof( cluster ) );

This suspicious code was found in Doom3 project by PVS-Studio static code analyzer.
Warning message is:
V512 A call of the 'memset' function will lead to underflow of the buffer '& cluster'. DoomDLL aasfile.cpp 1312

PVS-Studio is a static analyzer for detecting bugs in the source code of applications written in C, C++, C++11, C++/CX. Site: http://www.viva64.com/en/pvs-studio/