Robustness fix similar to d0d1392f964baa80f4e2e048e57fc012ffcd5802.
Root cause is presumably duplicate permissions.
Observed in BuildTriggerTest.testConfigureDownstreamProjectSecurity:
java.lang.ClassCastException: net.sf.json.JSONArray cannot be cast to java.lang.Boolean
	at hudson.security.AuthorizationMatrixProperty$DescriptorImpl.newInstance(AuthorizationMatrixProperty.java:150)
	at hudson.model.JobProperty.reconfigure(JobProperty.java:178)
	at hudson.model.JobProperty.reconfigure(JobProperty.java:75)
	at hudson.util.DescribableList.rebuild(DescribableList.java:171)
	at hudson.model.Job.doConfigSubmit(Job.java:1129)
	at hudson.model.AbstractProject.doConfigSubmit(AbstractProject.java:786)