[SPARK-12048][SQL] Prevent to close JDBC resources twice

Author: gcc <spark-src@condor.rhaag.ip>

Closes #10101 from rh99/master.

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/jdbc/JDBCRDD.scala

@@ -511,6 +511,7 @@ private[sql] class JDBCRDD(
       } catch {
         case e: Exception => logWarning("Exception closing connection", e)
       }
+      closed = true
     }
 
     override def hasNext: Boolean = {