fix stack corruption and LastError results in shlwapi PathCombine

[Vincent Povirk]

The length of the ANSI string doesn't necessarily match the length of
the Unicode string after conversion. We should be checking the return
value of MultiByteToWideChar to find out whether the converted string
fit.

Why aren't the 'strlenW(szTemp) + strlenW(lpszFile)' test and uses of
lstrcpynW in PathCombineW sufficient to avoid stack corruption?