PVS-Studio warning V547 for ReactOS

BOOL PrepareService(LPCTSTR ServiceName)
{
  DWORD LeftOfBuffer = sizeof(ServiceKeyBuffer) /
                       sizeof(ServiceKeyBuffer[0]);
  ....
  LeftOfBuffer -= _tcslen(SERVICE_KEY);
  ....
  LeftOfBuffer -= _tcslen(ServiceName);
  ....
  LeftOfBuffer -= _tcslen(PARAMETERS_KEY);
  ....
  if (LeftOfBuffer < 0)
  {
    DPRINT1("Buffer overflow for service name: '%s'\n",
            ServiceName);
    return FALSE;
  }
  ....
}

There are a lot of such fragments in ReactOS, but I cannot say for sure how critical they are.

This suspicious code was found in ReactOS project by PVS-Studio static code analyzer.
Warning message is:
V547 Expression 'LeftOfBuffer < 0' is always false. Unsigned type value is never < 0. svchost svchost.c 56

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/