bugfix to prevent crashes when  EEG sensor definition is rubbish (encountered in Neuromag data with EEG channels but no electrode location measurement)

git-svn-id: svn+ssh://svn.fcdonders.nl/home/svnroot/fieldtrip/trunk@8309 0cf7c7f0-3615-4144-b4e6-68da3bce3cd0