one more test for null-check in Builder#append(String)

Author: erosb

src/test/org/json/junit/JSONPointerTest.java

@@ -115,6 +115,11 @@ public void builderTest() {
         assertEquals("val", pointer.queryFrom(document));
     }
 
+    @Test(expected = NullPointerException.class)
+    public void nullToken() {
+        JSONPointer.builder().append(null);
+    }
+    
     @Test
     public void toStringEscaping() {
         JSONPointer pointer = JSONPointer.builder()