NPE in RecordAccumulator

I found this inadvertently while trying to create a system test to reproduce KAFKA-6003

java.lang.NullPointerException
        at org.apache.kafka.clients.producer.internals.RecordAccumulator.drain(RecordAccumulator.java:542)
        at org.apache.kafka.clients.producer.internals.Sender.sendProducerData(Sender.java:270)
        at org.apache.kafka.clients.producer.internals.Sender.run(Sender.java:238)
        at org.apache.kafka.clients.producer.internals.Sender.run(Sender.java:163)
        at java.lang.Thread.run(Thread.java:748)

The problem is with this line

if (first.hasSequence() && first.baseSequence() != transactionManager.nextBatchBySequence(first.topicPartition).baseSequence())

It is possible for the producer state to be reset (for instance if retries are expired), in which case the transaction manager will drop the in flight batches it is tracking. However, these batches will continue to be in the accumulator with a sequence, causing an NPE in the background thread on this line.

It would be better to drain the batches with the old Pid/Sequence in this case. Either they are accepted, or they will be returned to the user with an OutOfOrderSequenceException which is clearer.