Don't unnecessarily NUL-terminate wxCharBuffer contents.

Don't unnecessarily NUL-terminate wxCharBuffer contents.

wxCharBuffer already initializes the last byte of the buffer it allocates to 0
so there is no need to do it explicitly.

Also don't allocate an extra byte, wxCharBuffer already adds one to the length
passed to it for the trailing NUL.

See #13885.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73141 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775