Protect crawl converter against broken crawls

Some crawls don't contain all ASNs. As we only want to convert the crawl
and not add additional information we just skip adding stub info for
non-existant ASNs.

backend/crawler.py

@@ -199,7 +199,7 @@ def convert_crawl(crawl):
             if path:
                 net.nodes[path[-1]]['prefixes'].add(prefix)
                 for path_asn in path:
-                    if path_asn not in (path[-1], path[0]):
+                    if path_asn in net.nodes and path_asn not in (path[-1], path[0]):
                         net.nodes[path_asn]['stub'] = False
 
     # add neighbor count