bugfix - do not perform channel selection in source.avg.filter if channel labels are unknown. See http://bugzilla.fcdonders.nl/show_bug.cgi?id=2544


git-svn-id: svn+ssh://svn.fcdonders.nl/home/svnroot/fieldtrip/trunk@9447 0cf7c7f0-3615-4144-b4e6-68da3bce3cd0