Check list has members before trying to delete one

Closes #261 on github. Scrollable with no display list members
and no "lastline" will have cursor=None, and this must be
checked for explicitly before allowing a further deletion.

quicktill/stocklines.py

@@ -115,7 +115,7 @@ def __init__(self, stocklines=None,
                     headerline])
 
     def keypress(self, k):
-        if k == keyboard.K_CANCEL and self.s:
+        if k == keyboard.K_CANCEL and self.s and self.s.cursor is not None:
             line = self.s.dl.pop(self.s.cursor)
             self.s.redraw()
             td.s.add(line.userdata)