slirp: fix segv when init failed

Since commit f6c2e66ae8c8a, slirp uses an exit notifier to call
slirp_smb_cleanup. However, if init() failed, the notifier isn't added,
and removing it will fail:

==18447== Invalid write of size 8
==18447==    at 0x7EF2B5: notifier_remove (notify.c:32)
==18447==    by 0x48E80C: qemu_remove_exit_notifier (vl.c:2661)
==18447==    by 0x6A2187: net_slirp_cleanup (slirp.c:134)
==18447==    by 0x69419D: qemu_cleanup_net_client (net.c:338)
==18447==    by 0x69445B: qemu_del_net_client (net.c:401)
==18447==    by 0x6A2B81: net_slirp_init (slirp.c:366)
==18447==    by 0x6A4241: net_init_slirp (slirp.c:865)
==18447==    by 0x695C6D: net_client_init1 (net.c:1051)
==18447==    by 0x695F6E: net_client_init (net.c:1108)
==18447==    by 0x696DBA: net_init_netdev (net.c:1498)
==18447==    by 0x7F1F99: qemu_opts_foreach (qemu-option.c:1116)
==18447==    by 0x696E60: net_init_clients (net.c:1516)
==18447==  Address 0x0 is not stack'd, malloc'd or (recently) free'd

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
