Backport PR #2717: One liner to fix debugger printing stack traces when lines of context are larger than source. Correctly print stack traces when context is larger than lines of source. To test: ``` In [6]: def bar(): ...: return 20 ...: ``` Before: ``` In [7]: ipdb.runcall(bar) <ipython-input-6-d0ed3bd5e16c>(2)bar() 0 return 20 ``` And here's after: ``` In [6]: ipdb.runcall(bar) <ipython-input-5-d0ed3bd5e16c>(2)bar() 1 def bar(): ----> 2 return 20 ``` Replaces gh-2697. (Adds a test.)