Avoid the optimized mb_str() available in this build returning directly
a "char*" as it doesn't preserve the length of the string if it contains
NULs. Use mb_str(wxMBConv) overload instead which always returns the
buffer of the correct length.
Arguably, this is a problem of wxString API and maybe this optimized
mb_str() overload shouldn't be provided at all. OTOH the main reason for
wxUSE_UTF8_LOCALE_ONLY existence is optimization, so it seems a pity to
deliberately make the code less efficient when using it.