isisd: fix crash on "no router isis" (BZ#536)

The crash is due to threads accessing data that gets destroyed
during the removal of the configuration.

* isis_circuit.c: Destroy adjacencies to stop adjacency expiry thread.
  Stop PSNP threads.
* isisd.c: Change state of circuit back to INIT and reassign the
  circuit structure to isis->init_circ_list rather than destroying
  the circuit data structure. Stop SPF threads. Stop LSP generation
  threads.
* isisd.h: Add pointers to LSP threads into area structure in order to
  stop them in isisd.c
* isis_lsp.c: Store pointer to LSP thread in area structure.
* isis_pdu.c: Stop PDU generation for a circuit with a removed area.
* isis_pfpacket.c: Stop processing received PDUs for a circuit with a
  removed area.
diff --git a/isisd/isis_pfpacket.c b/isisd/isis_pfpacket.c
index 9e4165e..8a5c3ed 100644
--- a/isisd/isis_pfpacket.c
+++ b/isisd/isis_pfpacket.c
@@ -232,6 +232,10 @@
 			LLC_LEN, MSG_PEEK,
 			(struct sockaddr *) &s_addr, (socklen_t *) &addr_len);
 
+  if (!circuit->area) {
+    return ISIS_OK;
+  }
+
   if (bytesread < 0)
     {
       zlog_warn ("isis_recv_packet_bcast(): fd %d, recvfrom (): %s",