[FIXED JENKINS-15156] Initialize AbstractLazyLoadRunMap.dir for newly created jobs

Hi,

trying to debug [JENKINS-15156](https://issues.jenkins-ci.org/browse/JENKINS-15156) it seems that after creating a new job, the 'dir' member of its 'AbstractLazyLoadRunMap' of builds is null until Jenkins is restarted/reload. After the map's members get garbage-collected, reloading them fails because dir is null, giving
the missing builds.

This is a patch which for me seems to solve this.

This was originally sent to the jenkinsci-dev mailing list:
https://groups.google.com/forum/?fromgroups=#!topic/jenkinsci-dev/cJVw6pEHXKk

This is not really meant to be pulled verbatim. If anyone of the Jenkins developers likes to take up on this and fix this bug I'll be happy.

Regards,
Julian