Fix NullPointerException when executing query with --comments option

messages method was accesssing the notes member variable directly
instead of using the notes method, which will load the notes if not
already loaded.

Issue: 3210
Change-Id: I4b02f10d75070507cedbe9876a0f7c90944e0e2d

gerrit-server/src/main/java/com/google/gerrit/server/query/change/ChangeData.java

@@ -530,7 +530,7 @@ public class ChangeData {
   public List<ChangeMessage> messages()
       throws OrmException {
     if (messages == null) {
-      messages = cmUtil.byChange(db, notes);
+      messages = cmUtil.byChange(db, notes());
     }
     return messages;
   }