in creation of 'ql' questionlist variable: check whether questionlist is empty

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/exams@630 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
if file.copy() is asked to copy the same file multiple times to the
same target (in a single go), it always appends the file which leads
to invalid .Rnw files in the temporary folder -> hence duplication is
avoided now in xexams()



git-svn-id: svn://svn.r-forge.r-project.org/svnroot/exams@631 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
working on qti 2.1 num

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/exams@632 edb9625f-4e0d-4859-8d74-9fd3b1da38cb