PVS-Studio warning V557 for Miranda IM

const int idToIndex(const int id) const
{
  for(int i = 0; i < NR_BUTTONS; i++) {
    if(m_TbButtons[i].idCommand == id )
      return(i);
  }
  return(-1);
}

TBBUTTON CMenuBar::m_TbButtons[8] = {0};

void CMenuBar::invoke(const int id)
{
  const int index = idToIndex(id);
  ....
  if(index == 3 && hContact != 0) {
    ....
  } else if(index == 0) {
    ....
  } else
    hMenu = reinterpret_cast<HMENU>(m_TbButtons[index].dwData);
  ....
}

This suspicious code was found in Miranda IM project by PVS-Studio static code analyzer.
Warning message is:
V557 Array underrun is possible. The value of 'index' index could reach -1. controls.cpp 491

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