[SPARK-13838] [SQL] Clear variable code to prevent it to be re-evaluated in BoundAttribute

JIRA: https://issues.apache.org/jira/browse/SPARK-13838
## What changes were proposed in this pull request?

We should also clear the variable code in `BoundReference.genCode` to prevent it  to be evaluated twice, as we did in `evaluateVariables`.

## How was this patch tested?

Existing tests.

Author: Liang-Chi Hsieh <simonh@tw.ibm.com>

Closes #11674 from viirya/avoid-reevaluate.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/BoundAttribute.scala

@@ -65,7 +65,9 @@ case class BoundReference(ordinal: Int, dataType: DataType, nullable: Boolean)
       val oev = ctx.currentVars(ordinal)
       ev.isNull = oev.isNull
       ev.value = oev.value
-      oev.code
+      val code = oev.code
+      oev.code = ""
+      code
     } else if (nullable) {
       s"""
         boolean ${ev.isNull} = ${ctx.INPUT_ROW}.isNullAt($ordinal);