[SPARK-14578] [SQL] Fix codegen for CreateExternalRow with nested wide schema

## What changes were proposed in this pull request?

The wide schema, the expression of fields will be splitted into multiple functions, but the variable for loopVar can't be accessed in splitted functions, this PR change them as class member.

## How was this patch tested?

Added regression test.

Author: Davies Liu <davies@databricks.com>

Closes #12338 from davies/nested_row.

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

@@ -446,6 +446,8 @@ case class MapObjects private(
   override def genCode(ctx: CodegenContext, ev: ExprCode): String = {
     val javaType = ctx.javaType(dataType)
     val elementJavaType = ctx.javaType(loopVar.dataType)
+    ctx.addMutableState("boolean", loopVar.isNull, "")
+    ctx.addMutableState(elementJavaType, loopVar.value, "")
     val genInputData = inputData.gen(ctx)
     val genFunction = lambdaFunction.gen(ctx)
     val dataLength = ctx.freshName("dataLength")
@@ -466,9 +468,9 @@ case class MapObjects private(
     }
 
     val loopNullCheck = if (primitiveElement) {
-      s"boolean ${loopVar.isNull} = ${genInputData.value}.isNullAt($loopIndex);"
+      s"${loopVar.isNull} = ${genInputData.value}.isNullAt($loopIndex);"
     } else {
-      s"boolean ${loopVar.isNull} = ${genInputData.isNull} || ${loopVar.value} == null;"
+      s"${loopVar.isNull} = ${genInputData.isNull} || ${loopVar.value} == null;"
     }
 
     s"""
@@ -484,7 +486,7 @@ case class MapObjects private(
 
         int $loopIndex = 0;
         while ($loopIndex < $dataLength) {
-          $elementJavaType ${loopVar.value} =
+          ${loopVar.value} =
             ($elementJavaType)${genInputData.value}${itemAccessor(loopIndex)};
           $loopNullCheck
 

sql/core/src/test/scala/org/apache/spark/sql/execution/datasources/json/JsonSuite.scala

@@ -1664,4 +1664,19 @@ class JsonSuite extends QueryTest with SharedSQLContext with TestJsonData {
       )
     }
   }
+
+  test("wide nested json table") {
+    val nested = (1 to 100).map { i =>
+      s"""
+         |"c$i": $i
+       """.stripMargin
+    }.mkString(", ")
+    val json = s"""
+       |{"a": [{$nested}], "b": [{$nested}]}
+     """.stripMargin
+    val rdd = sqlContext.sparkContext.makeRDD(Seq(json))
+    val df = sqlContext.read.json(rdd)
+    assert(df.schema.size === 2)
+    df.collect()
+  }
 }