Fix selection of column in next/previous

When _sign is negative, it doesn't select the first column but the second. So
despite of selection the wrong column, it could crash if there is only one
column in the view.

review3241004
(transplanted from f05cc122d3fd52141386f8097d0fc3989b85abd4)

tryton/gui/window/view_form/view/list_gtk/editabletree.py

@@ -21,7 +21,7 @@ class TreeView(gtk.TreeView):
     def next_column(self, path, column=None, _sign=1):
         columns = self.get_columns()
         if column is None:
-            column = columns[-1 * _sign]
+            column = columns[-1 if _sign > 0 else 0]
         model = self.get_model()
         record = model.get_value(model.get_iter(path), 0)
         if _sign < 0: