Fix wxHtmlHelpData::SetTempDir() to behave correctly without trailing slash.

Fix wxHtmlHelpData::SetTempDir() to behave correctly without trailing slash.

The path passed to this function is always a directory, so use
wxFileName::AssignDir() instead of the default constructor of wxFileName which
considers the name to be a name of a file unless it ends with a path separator.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74696 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775