Fix IndexError for some inputs when creating un_chomsky_normal_form.

A sample sentence such as "That o'er the files and musters of the war
Have glow'd like plated Mars, now bend, now turn, The office and
devotion of their view Upon a tawny front." throws an error when parsed.
Merge pull request #2 from safehammad/fix-un-chomsky

Fix IndexError for some inputs when creating un_chomsky_normal_form.