improved sql parser, bug fixed for parent is null

src/main/java/com/alibaba/druid/sql/ast/SQLPartitionValue.java

@@ -33,6 +33,13 @@ public class SQLPartitionValue extends SQLObjectImpl {
     public List<SQLExpr> getItems() {
         return items;
     }
+    
+    public void addItem(SQLExpr item) {
+        if (item != null) {
+            item.setParent(this);
+        }
+        this.items.add(item);
+    }
 
     public Operator getOperator() {
         return operator;