isisd: refuse adjacencies with our own system ID

isisd would form an adjacency with another router despite the system IDs
being identical.  This would later cause an assertion failure like this:

  assertion=0x555555596db8 "isis_find_vertex (spftree->paths, id, vtype) == ((void *)0)",
  file=0x555555596c60 "isis_spf.c", line=515, function=0x555555597900 "isis_spf_add2tent") at log.c:619

which is caused by trying to add a path expected to not exist, but
suddenly colliding due to the duplicate system ID.

* isis_pdu.c: check for system ID collision on receiving Hello

isisd/isis_pdu.c

@@ -989,6 +989,13 @@ process_lan_hello (int level, struct isis_circuit *circuit, u_char * ssnpa)
         }
     }
 
+  if (!memcmp (hdr.source_id, isis->sysid, ISIS_SYS_ID_LEN))
+    {
+      zlog_warn ("ISIS-Adj (%s): duplicate system ID on interface %s",
+		 circuit->area->area_tag, circuit->interface->name);
+      return ISIS_WARNING;
+    }
+
   /*
    * Accept the level 1 adjacency only if a match between local and
    * remote area addresses is found