handle rare race condition on corpus closure

hyphe_backend/core.tac

@@ -2197,7 +2197,7 @@ class Memory_Structure(customJSONRPC):
         s = time.time()
         # Build links after at least one index if no more than 25000 pages in queue and...
         pages_crawled = yield self.db.check_pages(corpus)
-        if pages_crawled and self.corpora[corpus]['recent_changes'] and self.corpora[corpus]['pages_queued'] < 25000 and (
+        if pages_crawled and corpus in self.corpora and self.corpora[corpus]['recent_changes'] and self.corpora[corpus]['pages_queued'] < 25000 and (
             # pagesqueue is empty
             not self.corpora[corpus]['pages_queued'] or
             # links were not built since more than 8 times the time it takes