lsimodel: Only log top words that actually exist in <id2word>

In some pathological cases, we might try to log the top N words, even
though we haven't seen N words yet.  In these cases, we can just exit
the loop early.

Closes #3090.

gensim/models/lsimodel.py

@@ -670,7 +670,7 @@ def show_topic(self, topicno, topn=10):
         c = np.asarray(self.projection.u.T[topicno, :]).flatten()
         norm = np.sqrt(np.sum(np.dot(c, c)))
         most = matutils.argsort(np.abs(c), topn, reverse=True)
-        return [(self.id2word[val], 1.0 * c[val] / norm) for val in most]
+        return [(self.id2word[val], 1.0 * c[val] / norm) for val in most if val in self.id2word]
 
     def show_topics(self, num_topics=-1, num_words=10, log=False, formatted=True):
         """Get the most significant topics.