[SPARK-17465][SPARK CORE] Inappropriate memory management in...

[SPARK-17465][SPARK CORE] Inappropriate memory management in `org.apache.spark.storage.MemoryStore` may lead to memory leak

The expression like `if (memoryMap(taskAttemptId) == 0) memoryMap.remove(taskAttemptId)` in method `releaseUnrollMemoryForThisTask` and `releasePendingUnrollMemoryForThisTask` should be called after release memory operation, whatever `memoryToRelease` is > 0 or not.

If the memory of a task has been set to 0 when calling a `releaseUnrollMemoryForThisTask` or a `releasePendingUnrollMemoryForThisTask` method, the key in the memory map corresponding to that task will never be removed from the hash map.

See the details in [SPARK-17465](https://issues.apache.org/jira/browse/SPARK-17465).

Author: Xing SHI <shi-kou@indetail.co.jp>

Closes #15022 from saturday-shi/SPARK-17465.

core/src/main/scala/org/apache/spark/storage/memory/MemoryStore.scala

@@ -593,11 +593,11 @@ private[spark] class MemoryStore(
         val memoryToRelease = math.min(memory, unrollMemoryMap(taskAttemptId))
         if (memoryToRelease > 0) {
           unrollMemoryMap(taskAttemptId) -= memoryToRelease
-          if (unrollMemoryMap(taskAttemptId) == 0) {
-            unrollMemoryMap.remove(taskAttemptId)
-          }
           memoryManager.releaseUnrollMemory(memoryToRelease, memoryMode)
         }
+        if (unrollMemoryMap(taskAttemptId) == 0) {
+          unrollMemoryMap.remove(taskAttemptId)
+        }
       }
     }
   }