Unexpected flush causes NPE in connection pool

[akrambek]

Unpexxted flush causes NPE while doing a heartbeat.

2023-10-27 10:44:42 Caused by: java.lang.NullPointerException: Cannot invoke "io.aklivity.zilla.runtime.binding.kafka.internal.stream.KafkaClientConnectionPool$KafkaClientStream.doStreamSignalNow(long, int)" because "stream" is null
2023-10-27 10:44:42     at io.aklivity.zilla.runtime.binding.kafka/io.aklivity.zilla.runtime.binding.kafka.internal.stream.KafkaClientConnectionPool$KafkaClientSignaler.signalNow(KafkaClientConnectionPool.java:489)
2023-10-27 10:44:42     at io.aklivity.zilla.runtime.binding.kafka/io.aklivity.zilla.runtime.binding.kafka.internal.stream.KafkaClientGroupFactory$CoordinatorClient.doHeartbeatRequest(KafkaClientGroupFactory.java:3949)
2023-10-27 10:44:42     at io.aklivity.zilla.runtime.binding.kafka/io.aklivity.zilla.runtime.binding.kafka.internal.stream.KafkaClientGroupFactory$KafkaGroupStream.onStreamFlush(KafkaClientGroupFactory.java:1413)
2023-10-27 10:44:42     at io.aklivity.zilla.runtime.binding.kafka/io.aklivity.zilla.runtime.binding.kafka.internal.stream.KafkaClientGroupFactory$KafkaGroupStream.onStream(KafkaClientGroupFactory.java:1297)
2023-10-27 10:44:42     at io.aklivity.zilla.runtime.binding.kafka/io.aklivity.zilla.runtime.binding.kafka.internal.stream.KafkaClientGroupFactory.lambda$newStream$1(KafkaClientGroupFactory.java:366)
2023-10-27 10:44:42     at