fixup e0b5bd7: crash on endpoint.ListStale if replication-cursor-v1 bookmark present
```
cs@cstp:[~/zrepl/zrepl]: artifacts/zrepl-linux-amd64 zfs-abstraction release-stale --dry-run
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x9de971]
goroutine 1 [running]:
github.com/zrepl/zrepl/endpoint.listStaleFiltering(0xc00012b700, 0x5, 0x8, 0x0, 0x13ccb58)
/endpoint/endpoint_zfs_abstraction.go:736 +0x281
github.com/zrepl/zrepl/endpoint.ListStale(0xe3ae20, 0xc00026b740, 0x0, 0xe27c60, 0x13ccb58, 0xc0001d8690, 0x0, 0x0, 0x0, 0x1, ...)
/endpoint/endpoint_zfs_abstraction.go:698 +0x3fd
github.com/zrepl/zrepl/client.doZabsReleaseStale(0xe3ae20, 0xc00026b740, 0x13a28a0, 0xc000151be0, 0x0, 0x1, 0x5705b4, 0xc0002686e0)
/client/zfsabstractions_release.go:83 +0x1a0
github.com/zrepl/zrepl/cli.(*Subcommand).run(0x13a28a0, 0xc000264c80, 0xc000151be0, 0x0, 0x1)
/cli/cli.go:104 +0xf5
github.com/spf13/cobra.(*Command).execute(0xc000264c80, 0xc000151bd0, 0x1, 0x1, 0xc000264c80, 0xc000151bd0)
GOROOT/pkg/mod/github.com/spf13/cobra@v0.0.2/command.go:760 +0x2aa
github.com/spf13/cobra.(*Command).ExecuteC(0x13a43c0, 0x0, 0x0, 0x0)
GOROOT/pkg/mod/github.com/spf13/cobra@v0.0.2/command.go:846 +0x2ea
github.com/spf13/cobra.(*Command).Execute(...)
GOROOT/pkg/mod/github.com/spf13/cobra@v0.0.2/command.go:794
github.com/zrepl/zrepl/cli.Run()
/cli/cli.go:151 +0x2d
main.main()
/main.go:24 +0x20
```
Christian Schwarz
6f441c55dc