bugfix - ensure that tra exists prior to changing the channel scaling


git-svn-id: svn+ssh://svn.fcdonders.nl/home/svnroot/fieldtrip/trunk@8860 0cf7c7f0-3615-4144-b4e6-68da3bce3cd0
automatically synchronized identical files to revision 8860

git-svn-id: svn+ssh://svn.fcdonders.nl/home/svnroot/fieldtrip/trunk@8861 0cf7c7f0-3615-4144-b4e6-68da3bce3cd0