PVS-Studio warning V547 for CrashRpt

typedef std::basic_string<TCHAR> tstring;

int process_report(....)
{
  ....
  tstring sInDirName;
  ....
  size_t pos = sInDirName.rfind('\\');
  if(pos<0) // There is no back slash in path
  {
    sInDirName = _T("");
    sInFileName = szInput;
  }
  else
  {
    sInFileName = sInDirName.substr(pos+1);
    sInDirName = sInDirName.substr(0, pos);
  }
  ....
}

This suspicious code was found in CrashRpt project by PVS-Studio static code analyzer.
Warning message is:
V547 Expression 'pos < 0' is always false. Unsigned type value is never < 0. main.cpp 314

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