Check for buffer being big enough in wxPathOnly().

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