PVS-Studio warning V547 for Miranda IM

void Append( PCXSTR pszSrc, int nLength )
{
  ...
  UINT nOldLength = GetLength();
  if (nOldLength < 0)
  {
    // protects from underflow
    nOldLength = 0;
  }
  ...
}

This suspicious code was found in Miranda IM project by PVS-Studio static code analyzer.
Warning message is:
V547 Expression 'nOldLength < 0' is always false. Unsigned type value is never < 0. IRC mstring.h 229

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