Use wxString::To8BitData() instead of mb_str() to handle NULs correctly.

Use wxString::To8BitData() instead of mb_str() to handle NULs correctly.

In ANSI build wxString::mb_str() returns a pointer to the internal wxString
data directly instead of a buffer with a proper length, so it provides access
to the part of the string before the first embedded NUL only.

Use To8BitData() which always returns the buffer of the correct size in all
builds.

The open question remains whether mb_str() should be changed to return a (non
owned) buffer and not just a pointer in ANSI build. This would make
manipulating strings with embedded NULs safer but mb_str() would be less
efficient and less compatible.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66102 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775