修正重复rollback导致的问题

git-svn-id: http://code.alibabatech.com/svn/druid/trunk@860 b9813039-fb51-4c41-a8b9-e21c2acb5095

src/main/java/com/alibaba/druid/pool/PoolableConnection.java

@@ -587,6 +587,10 @@ public class PoolableConnection implements PooledConnection, Connection {
 
     @Override
     public void rollback() throws SQLException {
+        if (transactionInfo == null) {
+            return;
+        }
+    
         checkOpen();
 
         DruidAbstractDataSource dataSource = holder.getDataSource();