Avoid None.get exception in (rare?) case that no readLocks exist

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())
     }
     val writeLocks = synchronized {
       writeLocksByTask.remove(taskAttemptId).getOrElse(Seq.empty)