Markov: append a stop if we have nothing to append from a chain

somehow a chain led us down a path where there are no values for
the keys in the chain. if that happens, just abort.

i'm not quite sure how this could happen

modules/Markov.py

@@ -284,6 +284,8 @@ class Markov(Module):
             else:
                 if len(gen_words) < min_size and len(filter(lambda a: a != self.stop, key_hits)) > 0:
                     gen_words.append(random.choice(filter(lambda a: a != self.stop, key_hits)))
+                elif len(key_hits) <= 0:
+                    gen_words.append(self.stop)
                 else:
                     gen_words.append(random.choice(key_hits))