bpo-28528: Fix pdb.checkline() attribute error when 'curframe' is None. (GH-25438) (#26050)

Co-authored-by: Thomas Kluyver <takowl@gmail.com>
(cherry picked from commit 8563a7052ccd98e6a381d361664ce567afd5eb6e)

Co-authored-by: Erlend Egeberg Aasland <erlend.aasland@innova.no>
3 files changed