add assert

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1175397 13f79535-47bb-0310-9956-ffa450edef68

modules/join/src/java/org/apache/lucene/search/join/BlockJoinQuery.java

@@ -322,6 +322,9 @@ public class BlockJoinQuery extends Query {
         return parentDoc = NO_MORE_DOCS;
       }
 
+      // Every parent must have at least one child:
+      assert parentTarget != 0;
+
       final int prevParentDoc = parentBits.prevSetBit(parentTarget-1);
 
       //System.out.println("  rolled back to prevParentDoc=" + prevParentDoc + " vs parentDoc=" + parentDoc);