PVS-Studio warning V547 for WinMerge

void CMergeDoc::Computelinediff(....)
{
  ....
  vector<wdiff*>::size_type i;
  ....
  for (i=worddiffs.size() - 1; i>=0; --i)
  {
    const wdiff * diff = worddiffs[i];
    if (end1 == -1 && diff->end[0] != -1)
      end1 = diff->end[0];
    if (end2 == -1 && diff->end[1] != -1)
      end2 = diff->end[1];
    if (end1 != -1 && end2 != -1)
      break; // found both
  }
  ....
}

This suspicious code was found in WinMerge project by PVS-Studio static code analyzer.
Warning message is:
V547 Expression 'i >= 0' is always true. Unsigned type value is always >= 0. mergedoclinediffs.cpp 282

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