glade crashes if you drag the "< define a new column >" row of List Stores & Tree Stores when no columns are defined yet

Binary package hint: glade-3

Steps to reproduce this:
* start Glade in GtkBuilder mode and create a "List Store" tree model
* make sure the new list store is selected in the object tree
* in the properties editor, there is a treeview to add columns to the list store; do NOT add any columns
* drag the row that reads "< define a new column >" to the empty space below it in the treeview

This makes Glade "crash" with the following line:

GladeUI-GTK:ERROR:glade-column-types.c:456:columns_changed_idle: assertion failed: (columns)

Doctormo first discovered this because he was using a wacom tablet, and it's easy then to accidentally drag something when you just try to click

Tested by me and happens consistently on an up-to-date maverick (64-bits) & natty (32-bits).

The same crash can be invoked when editing a "tree store".