Check for buffer being big enough in wxPathOnly().

Just return NULL or empty string if the input path is too long. This is
probably not ideal but it fixes a buffer overflow and all this code needs to
be rewritten to use wxFileName() anyhow so it's not worth doing anything more
at this moment.

Closes #15302.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74455 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775