Correctly convert wxPrintf() to a buffer even when it doesn't fit.

Correctly convert wxPrintf() to a buffer even when it doesn't fit.

ConvertStringToBuf() helper function was defined incorrectly for converting
wxString to a char* buffer as it didn't fill the buffer at all if the string
didn't fit into it entirely instead of putting as much of the string into it
as possible as was already done for the conversion to wchar_t* buffer. This
broke wxSprintf()-related functions in when the ASCII output buffer was not
big enough as it was not filled at all.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65691 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775