Always set edc->dc = NULL in tman_unhighlight

Fixes bug where shutting down the join editor without first closing the
trace display causes a crash the next time an attempt is made to view a
trace.


git-svn-id: https://staden.svn.sourceforge.net/svnroot/staden/staden/trunk@3116 ba6f2710-ca9f-427c-89d3-5bf716dbc8e1