fixes a panic where prefetch could try to operate on a nil object (#1499)
fixes a panic where prefetch could try to operate on a nil object (#1499)

Signed-off-by: Edward Welch <4e1cbdeb5d169ce17df53d96f1e78e8fa81360a1@grafana.com>
grafana/loki,grafana/loki,grafana/loki,grafana/loki,grafana/loki,grafana/loki