bug fixed : after real connection closed, closePoolableStatementError

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

@@ -81,6 +81,10 @@ public class PoolableConnection implements PooledConnection, Connection {
     }
 
     void closePoolableStatement(PoolablePreparedStatement stmt) throws SQLException {
+        if (this.isClosed()) {
+            return;
+        }
+        
         PreparedStatement rawStatement = stmt.getRawPreparedStatement();
 
         try {
@@ -88,6 +92,10 @@ public class PoolableConnection implements PooledConnection, Connection {
         } catch (SQLException ex) {
             LOG.error("clear parameter error", ex);
         }
+        
+        if (holder == null) {
+            return;
+        }
 
         if (holder.isPoolPreparedStatements()) {
             stmt.clearResultSet();