Reimplemented xccdf_policy_get_readable_item_{title,description} to prevent SIGSEGV

This is already fixed in openscap, see 41fa3103cf7293ddc4bb891766ef9dbf5c9921fe.
scap-workbench reimplements these functions to allow readable
descriptions with XHTML tags in the future. Also to prevent crashes with
older versions of openscap.