update_connections: do not run mark_list_depth on invalid pipeline
update_connections: do not run mark_list_depth on invalid pipeline

This was triggered by the upgrade code

gui/pipeline_view.py:
 - update_connections:
   Check if valid before calling mark_list_depth

minesense/VisTrails,celiafish/VisTrails,minesense/VisTrails,VisTrails/VisTrails,VisTrails/VisTrails,hjanime/VisTrails,celiafish/VisTrails,hjanime/VisTrails,celiafish/VisTrails,Nikea/VisTrails,celiafish/VisTrails,Nikea/VisTrails,hjanime/VisTrails,Nikea/VisTrails,VisTrails/VisTrails,minesense/VisTrails,Nikea/VisTrails,VisTrails/VisTrails,minesense/VisTrails,hjanime/VisTrails,minesense/VisTrails,hjanime/VisTrails,VisTrails/VisTrails