[JENKINS-21576] NPE NullPointerException in DiskUsagePlugin

- Check for null added to DiskUsageUtil.controlWorkspaceExceedSize() to prevent NullPointerException. Method project.getProperty() will return null if property of such class is not found.
- Indentation fixed in controlWorkspaceExceedSize().
- Typo in method name controlorkspaceExceedSize() fixed (missing 'W' added).