Adds Precondition to TopicModel.update(Matrix)

This will catch poorly behaved clients who request less than one thread on TopicModel instantiation, but then attempt to use the multithreaded update method.

core/src/main/java/org/apache/mahout/clustering/lda/cvb/TopicModel.java

@@ -16,6 +16,7 @@
  */
 package org.apache.mahout.clustering.lda.cvb;
 
+import com.google.common.base.Preconditions;
 import com.google.common.collect.Lists;
 import org.apache.hadoop.conf.Configurable;
 import org.apache.hadoop.conf.Configuration;
@@ -329,6 +330,8 @@ public Vector infer(Vector original, Vector docTopicPrior, double minRelPerplexi
   }
 
   public void update(Matrix docTopicCounts) {
+    Preconditions.checkState(updaters.length > 0,
+        "Unable to update model; No threads requested during TopicModel instantiation");
     for(int x = 0; x < numTopics; x++) {
       updaters[x % updaters.length].update(x, docTopicCounts.viewRow(x));
     }