Potential fix on 1.8

[Seamus3131]

Hello,

influx version 1.8.10
we got a crash when calling epoch.StartWrite()
influxdb/tsdb/store.go line 794 : method DeleteShard
"invalid address or nil pointer dereference"

We added a nil check on epoch variable doing that influx no more crashes.
Here is the code

`

if epoch != nil {
	guards, gen := epoch.StartWrite()
	defer epoch.EndWrite(gen)
	if guards != nil {
		// wait for any guards before closing the shard
		for _, guard := range guards {
			guard.Wait()
		}
	}
} else {
	s.Logger.Warn("Epoch is nil",
					logger.Shard(sh.ID()))
}
// Close the shard.
if err := sh.Close(); err != nil {
	return err
}
// Remove the on-disk shard data.
if err := os.RemoveAll(sh.path); err != nil {
	return err
}
return os.RemoveAll(sh.walPath)

`

My question is the following,

In which case
epoch := s.epochs[shardID] --> influxdb/tsdb/store.go line 725 : method DeleteShard
can bring a nil pointer?

Is it fine, to do nothing for epoch if nil (this I am pretty sure) and continue with the deletion of shard anyway ?
Can it leads to massive issues afterward ?