[SPARK-16599][CORE] java.util.NoSuchElementException: None.get at at...

[SPARK-16599][CORE] java.util.NoSuchElementException: None.get at at org.apache.spark.storage.BlockInfoManager.releaseAllLocksForTask

## What changes were proposed in this pull request?

Avoid None.get exception in (rare?) case that no readLocks exist
Note that while this would resolve the immediate cause of the exception, it's not clear it is the root problem.

## How was this patch tested?

Existing tests

Author: Sean Owen <sowen@cloudera.com>

Closes #17290 from srowen/SPARK-16599.

core/src/main/scala/org/apache/spark/storage/BlockInfoManager.scala

@@ -23,7 +23,7 @@ import scala.collection.JavaConverters._
 import scala.collection.mutable
 import scala.reflect.ClassTag
 
-import com.google.common.collect.ConcurrentHashMultiset
+import com.google.common.collect.{ConcurrentHashMultiset, ImmutableMultiset}
 
 import org.apache.spark.{SparkException, TaskContext}
 import org.apache.spark.internal.Logging
@@ -340,7 +340,7 @@ private[storage] class BlockInfoManager extends Logging {
     val blocksWithReleasedLocks = mutable.ArrayBuffer[BlockId]()
 
     val readLocks = synchronized {
-      readLocksByTask.remove(taskAttemptId).get
+      readLocksByTask.remove(taskAttemptId).getOrElse(ImmutableMultiset.of[BlockId]())
     }
     val writeLocks = synchronized {
       writeLocksByTask.remove(taskAttemptId).getOrElse(Seq.empty)