PVS-Studio warning V620 for Xerces

const XMLCh *DOMNormalizer::integerToXMLCh(unsigned int i) const
{
  XMLCh *buf = (XMLCh*)
               fMemoryManager->allocate(15 * sizeof(XMLCh));
  XMLCh *pos = buf + sizeof(buf) - sizeof(XMLCh);
  ....
}

This suspicious code was found in Xerces project by PVS-Studio static code analyzer.
Warning message is:
V620 It's unusual that the expression of sizeof(T) kind is being subtracted from the pointer to T type. domnormalizer.cpp 277

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