mandos-client: Fix some bugs on error conditions.

* plugins.d/mandos-client.c (run_network_hooks): Init "dirent" to NULL
                                                 and, later, always
                                                 free() it unless
                                                 scandirat() or
                                                 scandir() failed.
  (main): Fix free() of uninitalized memory in case scandirat() or
          scandir() of "/sys/class/net" failed.  Also, when cleaning
          up, even if GPGME temp directory is empty, do
          free(direntries) and remove the directory.