WAL checkpoint causes panics

[simonswine]

Describe the bug

During WAL checkpointing I have encountered a panic

Expected behavior

• Do not panic

Environment:

• Infrastructure: At Grafana Labs
• Version: weekly-k102-6b9361d0

Screenshots, Promtail config, or terminal output*

level=info ts=2022-06-27T14:36:10.692694002Z caller=checkpoint.go:615 msg="starting checkpoint"
level=info ts=2022-06-27T14:36:10.693230616Z caller=checkpoint.go:340 msg="attempting checkpoint for" dir=/loki/wal/checkpoint.006820
level=info ts=2022-06-27T14:36:10.965299281Z caller=checkpoint.go:573 msg="checkpoint done" time=263.476585ms
panic: runtime error: index out of range [1540] with length 1540

goroutine 1178 [running]:
github.com/grafana/loki/pkg/ingester.newStreamsIterator({0x295c800, 0xc000e06580})
        /src/enterprise-logs/vendor/github.com/grafana/loki/pkg/ingester/checkpoint.go:216 +0x24e
github.com/grafana/loki/pkg/ingester.(*ingesterSeriesIter).Iter(0x1f2a73a)
        /src/enterprise-logs/vendor/github.com/grafana/loki/pkg/ingester/checkpoint.go:189 +0x25
github.com/grafana/loki/pkg/ingester.(*Checkpointer).PerformCheckpoint(0xc0109f9f78)
        /src/enterprise-logs/vendor/github.com/grafana/loki/pkg/ingester/checkpoint.go:577 +0x25f
github.com/grafana/loki/pkg/ingester.(*Checkpointer).Run(0xc0109f9f78)
        /src/enterprise-logs/vendor/github.com/grafana/loki/pkg/ingester/checkpoint.go:616 +0x21c
github.com/grafana/loki/pkg/ingester.(*walWrapper).run(0xc000b44690)
        /src/enterprise-logs/vendor/github.com/grafana/loki/pkg/ingester/wal.go:166 +0x21c
created by github.com/grafana/loki/pkg/ingester.(*walWrapper).Start
        /src/enterprise-logs/vendor/github.com/grafana/loki/pkg/ingester/wal.go:102 +0x6f