fixed algorithm

git-svn-id: https://svn.apache.org/repos/asf/maven/shared/trunk@1475947 13f79535-47bb-0310-9956-ffa450edef68
avoid NPE if parent has no modules

git-svn-id: https://svn.apache.org/repos/asf/maven/shared/trunk@1475951 13f79535-47bb-0310-9956-ffa450edef68