Fix wrong resulting string length in UTF-16 to wchar_t conversion.

Don't optimize the returned length for surrogate-less case, this does save a
pass of the string but at the price of returning a wrong result, which is not
worth it, just compute the really required length exactly.

Closes #16298.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76618 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775