add a check for IllegalArgumentException

git-svn-id: https://wala.svn.sourceforge.net/svnroot/wala/trunk@2304 f5eafffb-2e1d-0410-98e4-8ec43c5233c4

com.ibm.wala.core/src/com/ibm/wala/util/intset/SparseIntSet.java

@@ -349,6 +349,9 @@ public class SparseIntSet implements IntSet {
   }
 
   public IntSet intersection(IntSet that) {
+    if (that == null) {
+      throw new IllegalArgumentException("that == null");
+    }
     if (that instanceof SparseIntSet) {
       MutableSparseIntSet temp = MutableSparseIntSet.make(this);
       temp.intersectWith((SparseIntSet) that);