Fix library bug causes code crash

[CameronLonsdale]

You can trigger it by writing 'A' all the way to the end of the first line in the first example file.

[CameronLonsdale]

The issue has been documented here: urwid/urwid#322
I think the best solution going forward would be to encode or prevent unprintable characters from being rendered. Since the scope of MTP is to deal with printable text, this is an acceptable compromise.