fix IndexError for single-path satin columns (fixes #366)

lib/elements/satin_column.py

@@ -325,6 +325,9 @@ class SatinColumn(EmbroideryElement):
             self.fatal(_("satin column: object %s has a fill (but should not)") % node_id)
 
         if not self.rungs:
+            if len(self.rails) < 2:
+                self.fatal(_("satin column: object %(id)s has too few paths.  A satin column should have at least two paths (the rails).") % dict(id=node_id))
+
             if len(self.rails[0]) != len(self.rails[1]):
                 self.fatal(_("satin column: object %(id)s has two paths with an unequal number of points (%(length1)d and %(length2)d)") %
                            dict(id=node_id, length1=len(self.rails[0]), length2=len(self.rails[1])))