Fix return value of wxMBConvUTF8::ToWChar() when not using MAP_INVALID_UTF8_NOT.

Fix return value of wxMBConvUTF8::ToWChar() when not using MAP_INVALID_UTF8_NOT.

wxMBConvUTF8::ToWChar() was off by 1 when the input length was explicitly
specified, the extra NUL should only be added in the implicit length case.

This bug didn't occur for the default wxMBConvUTF8 object as it simply
forwarded to the base class wxMBConvStrictUTF8 implementation but it happened
when MAP_INVALID_UTF8_TO_OCTAL or MAP_INVALID_UTF8_TO_PUA was used.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68694 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775