fix boundary checking problem similiar to the one fixed for bug report 4290
Actually this wouldn't throw an ArrayIndexOutOuBounds- but a
BuildException as FileList.getFiles barfs out on empty lists, but that
way it is more consistent.
git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@271456 13f79535-47bb-0310-9956-ffa450edef68