title: Jenkins with Xvfb plugin: got "Prefix string too short" error on empty build
body: <p>I created an empty item, check "Start Xvfb before the build, and shut it down after.", and build just failed with error.</p>
  
  <pre><code>Building in workspace /var/lib/jenkins/jobs/test/workspace
  FATAL: Prefix string too short
  java.lang.IllegalArgumentException: Prefix string too short
      at java.io.File.createTempFile(File.java:1978)
      at hudson.FilePath$18.invoke(FilePath.java:1404)
      at hudson.FilePath$18.invoke(FilePath.java:1401)
      at hudson.FilePath.act(FilePath.java:989)
      at hudson.FilePath.act(FilePath.java:967)
      at hudson.FilePath.createTempDir(FilePath.java:1401)
      at org.jenkinsci.plugins.xvfb.XvfbBuildWrapper.launchXvfb(XvfbBuildWrapper.java:484)
      at org.jenkinsci.plugins.xvfb.XvfbBuildWrapper.setUp(XvfbBuildWrapper.java:594)
      at hudson.model.Build$BuildExecution.doRun(Build.java:154)
      at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:533)
      at hudson.model.Run.execute(Run.java:1718)
      at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
      at hudson.model.ResourceController.execute(ResourceController.java:89)
      at hudson.model.Executor.run(Executor.java:240)
  </code></pre>
  
  <p>What goes wrong?</p>
  
  <ul>
  <li>Ubuntu Server 14.10</li>
  <li>Jenkins 1.597</li>
  <li>xvfb plugin 1.0.13</li>
  <li>Xvfb executable path: /usr/bin/Xvfb</li>
  </ul>