Don't cache incorrect length in wxString::assign(char*, size_t).

Don't cache incorrect length in wxString::assign(char*, size_t).

The length of the string here is specified in bytes and is not the same as the
string length for non-ASCII UTF-8 strings, so don't cache it as the string
length.

Just invalidate the cached length instead as we simply don't know what the
real length of the string is going to be.

See #14130.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70987 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775