plugin-runner: Release memory and close FD's correctly in all cases.

* plugins.d/mandos-client.c (main): Init "direntries" to NULL.  Close
                                    "dir_fd" at fallback label.  Do
                                    not free "direntries" if
                                    scandirat() or scandir() failed.
                                    Do free(direntries) at fallback
                                    label.