Ensure wxCharTypeBuffer data is NUL-terminated after extend() call.

Ensure wxCharTypeBuffer data is NUL-terminated after extend() call.

As wxCharTypeBuffer ctor taking the length NUL-terminates the buffer, it may
be expected that extend() does the same but it did not. Do add the NUL at the
end for consistency, even though it's not really needed for the existing code
using extend() in wxWidgets itself.

Closes #13885.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70417 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775