Fix multiple inplementation errors including the incorrect usage of S_FALSE.
Make this function behave like the Windows 8 implementation and
the according MSDN specification.
- return S_OK if the extension was NOT found
- if no extension was found, ppszExt must point to the string's terminating null
- return E_INVALIDARG if pszPath is not null-terminated within the cchPath range
- return E_INVALIDARG if pszPath is NULL
- return E_INVALIDARG if ppszExt is NULL
- return E_INVALIDARG if cchPath is Zero
- return E_NOTIMPL instead of S_OK in PathPathCchFindExtensionW()
Also extended/fixed the TestPathCchFindExtension ctest
