[media] mantis: fix silly crash case

If we set mantis->fe to NULL on an error its not a good idea to then try
passing NULL to the unregister paths and oopsing really.

Resolves-bug: https://bugzilla.kernel.org/show_bug.cgi?id=16473

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/dvb/mantis/mantis_dvb.c

@@ -248,8 +248,10 @@ int __devinit mantis_dvb_init(struct mantis_pci *mantis)
 err5:
 	tasklet_kill(&mantis->tasklet);
 	dvb_net_release(&mantis->dvbnet);
-	dvb_unregister_frontend(mantis->fe);
-	dvb_frontend_detach(mantis->fe);
+	if (mantis->fe) {
+		dvb_unregister_frontend(mantis->fe);
+		dvb_frontend_detach(mantis->fe);
+	}
 err4:
 	mantis->demux.dmx.remove_frontend(&mantis->demux.dmx, &mantis->fe_mem);