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