Fixing a SIOOBE encountered during Windows-jdk11 tests (but not reproducible locally).

Amends code added in https://github.com/jenkinsci/jenkins/pull/2082.
java.lang.RuntimeException: Unexpected issues encountered during cleanUp: offset 0, count -1, length 5
	at jenkins.model.Jenkins.cleanUp(Jenkins.java:3319)
	at org.jvnet.hudson.test.JenkinsRule.after(JenkinsRule.java:495)
	at org.jvnet.hudson.test.JenkinsRule$1.evaluate(JenkinsRule.java:565)
	at org.junit.internal.runners.statements.FailOnTimeout$CallableStatement.call(FailOnTimeout.java:298)
	at org.junit.internal.runners.statements.FailOnTimeout$CallableStatement.call(FailOnTimeout.java:292)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at java.base/java.lang.Thread.run(Thread.java:834)
Caused by: java.lang.StringIndexOutOfBoundsException: offset 0, count -1, length 5
	at java.base/java.lang.String.checkBoundsOffCount(String.java:3304)
	at java.base/java.lang.String.<init>(String.java:465)
	at hudson.TcpSlaveAgentListener$PingAgentProtocol.connect(TcpSlaveAgentListener.java:416)
	at hudson.TcpSlaveAgentListener.shutdown(TcpSlaveAgentListener.java:196)
	at jenkins.model.Jenkins._cleanUpShutdownTcpSlaveAgent(Jenkins.java:3548)
	at jenkins.model.Jenkins.cleanUp(Jenkins.java:3294)
	... 6 more