Fix potential NPE in queries with --dependencies (descendants)

If an entry in patch_set_ancestors references a non-existing
entry in the change table, an NPE would be thrown and the query
would be aborted.  Instead, skip over that entry so the query
still works.

Change-Id: I985c0d0aec926e991eb3b140186c7de0f598b7ea

gerrit-server/src/main/java/com/google/gerrit/server/events/EventFactory.java

@@ -261,6 +261,12 @@ public class EventFactory {
             continue;
           }
           final Change c = db.changes().get(p.getId().getParentKey());
+          if (c == null) {
+            log.error("Error while generating the list of descendants for"
+                + " revision " + revId.get() + ": Cannot find Change entry in"
+                + " database for " + p.getId().getParentKey());
+            continue;
+          }
           ca.neededBy.add(newNeededBy(c, p));
         }
       }