Fix seg fault for sentence ending in PA

I encountered issue #3, saw it was already reported, and here's my attempt at a fix.

* I'm not too skilled at Lojban or all that knowledgable about its grammar
* I'm not that certain of how this NFA/DFA state machine is set up

So if this change is totally wrong please let me know. The segmentation fault (which happened when very last input token was in PA) starts at https://github.com/lojban/jbofihe/blob/665e109/categ.c#L383 because `last_ok_token` is `NULL`.