## What changes were proposed in this pull request?

In Average.scala, it has
```
  override lazy val evaluateExpression = child.dataType match {
    case DecimalType.Fixed(p, s) =>
      // increase the precision and scale to prevent precision loss
      val dt = DecimalType.bounded(p + 14, s + 4)
      Cast(Cast(sum, dt) / Cast(count, dt), resultType)
    case _ =>
      Cast(sum, resultType) / Cast(count, resultType)
  }

  def setChild (newchild: Expression) = {
    child = newchild
  }

```
It is possible that  Cast(count, dt), resultType) will make the precision of the decimal number bigger than 38, and this causes over flow.  Since count is an integer and doesn't need a scale, I will cast it using DecimalType.bounded(38,0)
## How was this patch tested?
In DataFrameSuite, I will add a test case.

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: Huaxin Gao <huaxing@us.ibm.com>

Closes #19496 from huaxingao/spark-22271.

(cherry picked from commit 28f9f3f22511e9f2f900764d9bd5b90d2eeee773)
Signed-off-by: gatorsmile <gatorsmile@gmail.com>

# Conflicts:
#	sql/core/src/test/scala/org/apache/spark/sql/DataFrameSuite.scala