* perl/Amanda/Changer/disk.pm: Return slot with invalid slot.
* perl/Amanda/Changer/robot.pm: Return slot with invalid slot.
* server-src/amtape.pl: Continue on invalid slot.


git-svn-id: https://amanda.svn.sourceforge.net/svnroot/amanda/amanda/trunk@5136 a8d146d6-cc15-0410-8900-af154a0219e0