Terminal: don't let Readline return [0, 0]

Some of the tests from "spec/commands/ls_spec.rb" were failing on
Rubinius and 1.9.2. The problems was introduced by this commit:

  (Tweak and Refactor Pry::Terminal)
  fc522f837a59fe2003c667a48860529610d6f7d8

The problem is that `Readline.get_screen_size` can return `[0, 0]`.
That's not good, because it breaks a couple of our tests (table output
stuff). But now...

          .o( ...it is fixed. Twit-twoo! )
    \{0,0}/