Ensure that wxFileName::GetTempDir() doesn't return trailing slashes.

Ensure that wxFileName::GetTempDir() doesn't return trailing slashes.

Sanitize the value returned by GetTempDir() to ensure that it doesn't have any
trailing path separators. This happened at least under OS X where standard
TMPDIR has a trailing slash and was inconsistent with the behaviour under the
other platforms.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64638 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775