Pdb.checkline() does a hasattr() check to protect against self.curframe not existing. self.curframe can also be None (if self.forget() or self.reset() was called), but checkline() does not handle this.

The attached patch treats self.curframe == None as equivalent to the attribute being absent.

Background:
http://bugs.python.org/issue9230
https://github.com/ipython/ipython/issues/10028

(Georg, I've nosy-listed you as I saw your name on a couple of similar issues; I hope you don't mind)