[SPARK-18528][SQL] Fix a bug to initialise an iterator of aggregation buffer

## What changes were proposed in this pull request?
This pr is to fix an `NullPointerException` issue caused by a following `limit + aggregate` query;
```
scala> val df = Seq(("a", 1), ("b", 2), ("c", 1), ("d", 5)).toDF("id", "value")
scala> df.limit(2).groupBy("id").count().show
WARN TaskSetManager: Lost task 0.0 in stage 9.0 (TID 8204, lvsp20hdn012.stubprod.com): java.lang.NullPointerException
at org.apache.spark.sql.catalyst.expressions.GeneratedClass$GeneratedIterator.agg_doAggregateWithKeys$(Unknown Source)
at org.apache.spark.sql.catalyst.expressions.GeneratedClass$GeneratedIterator.processNext(Unknown Source)
```
The root culprit is that [`$doAgg()`](https://github.com/apache/spark/blob/master/sql/core/src/main/scala/org/apache/spark/sql/execution/aggregate/HashAggregateExec.scala#L596) skips an initialization of [the buffer iterator](https://github.com/apache/spark/blob/master/sql/core/src/main/scala/org/apache/spark/sql/execution/aggregate/HashAggregateExec.scala#L603); `BaseLimitExec` sets `stopEarly=true` and `$doAgg()` exits in the middle without the initialization.

## How was this patch tested?
Added a test to check if no exception happens for limit + aggregates in `DataFrameAggregateSuite.scala`.

Author: Takeshi YAMAMURO <linguin.m.s@gmail.com>

Closes #15980 from maropu/SPARK-18528.

1 files changed, 10 insertions, 0 deletions

diff --git a/sql/core/src/main/java/org/apache/spark/sql/execution/BufferedRowIterator.java b/sql/core/src/main/java/org/apache/spark/sql/execution/BufferedRowIterator.java
index 086547c793..730a4ae8d5 100644
--- a/sql/core/src/main/java/org/apache/spark/sql/execution/BufferedRowIterator.java
+++ b/sql/core/src/main/java/org/apache/spark/sql/execution/BufferedRowIterator.java
@@ -70,6 +70,16 @@ public abstract class BufferedRowIterator {
   }
 
   /**
+   * Returns whether this iterator should stop fetching next row from [[CodegenSupport#inputRDDs]].
+   *
+   * If it returns true, the caller should exit the loop that [[InputAdapter]] generates.
+   * This interface is mainly used to limit the number of input rows.
+   */
+  protected boolean stopEarly() {
+    return false;
+  }
+
+  /**
    * Returns whether `processNext()` should stop processing next row from `input` or not.
    *
    * If it returns true, the caller should exit the loop (return from processNext()).