PVS-Studio warning V521 for VirtualBox

void Appliance::i_importMachines(....)
{
  ....
  /* Iterate through all virtual systems of that appliance */
  size_t i = 0;
  for (it = reader.m_llVirtualSystems.begin(),
       it1 = m->virtualSystemDescriptions.begin();
       it != reader.m_llVirtualSystems.end(),         //<==
       it1 != m->virtualSystemDescriptions.end();
       ++it, ++it1, ++i)
  {....}
  ....
}

This suspicious code was found in VirtualBox project by PVS-Studio static code analyzer.
Warning message is:
V521 Such expressions using the ',' operator are dangerous. Make sure the expression is correct. applianceimplimport.cpp 3943

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/