`rhcos.upgrade.from-ocp-rhcos` failed with golang panic

Bug Report

Environment

What operating system is being used to run coreos-assembler?

RHCOS

What operating system is being assembled?

RHCOS 4.10

Is coreos-assembler running in Podman or Docker?

OCP 4 pod, unprivileged

Expected Behavior

Test succeeds

Actual Behavior

**--- FAIL: rhcos.upgrade.from-ocp-rhcos (3.15s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x78 pc=0x1a13670]

goroutine 193 [running]:
github.com/coreos/mantle/harness.tRunner.func1(0xc0003658c0)
	/root/containerbuild/mantle/harness/harness.go:470 +0x2fe
panic(0x1e15d80, 0x347a120)
	/usr/lib/golang/src/runtime/panic.go:965 +0x1b9
github.com/coreos/mantle/kola/tests/rhcos.downloadLatestReleasedRHCOS(0x22ab86a, 0x4, 0x0, 0x0, 0x0, 0x0)
	/root/containerbuild/mantle/kola/tests/rhcos/upgrade.go:376 +0x850
github.com/coreos/mantle/kola/tests/rhcos.rhcosUpgradeFromOcpRhcos(0xc0003658c0, 0x26489a8, 0xc000b9a030, 0x0, 0x0, 0x0, 0x1)
	/root/containerbuild/mantle/kola/tests/rhcos/upgrade.go:212 +0x1ab
github.com/coreos/mantle/kola.runTest(0xc0003658c0, 0xc0000c0820, 0x7ffc2848222a, 0xb, 0x263d418, 0xc00031f290)
	/root/containerbuild/mantle/kola/harness.go:1333 +0xb51
github.com/coreos/mantle/kola.runProvidedTests.func1(0xc0003658c0)
	/root/containerbuild/mantle/kola/harness.go:632 +0x7f
github.com/coreos/mantle/harness.tRunner(0xc0003658c0, 0xc000259500)
	/root/containerbuild/mantle/harness/harness.go:512 +0x104
created by github.com/coreos/mantle/harness.(*H).RunTimeout
	/root/containerbuild/mantle/harness/harness.go:557 +0x38e
qemu-system-x86_64:qemu-system-x86_64:  terminating on signal 15 from pid 3053 ()terminating on signal 15 from pid 3053 ()**

Reproduction Steps

1. Build RHCOS 4.9 in Prow
2. Run rhcos.upgrade.from-ocp-rhcos

Other Information

This was seen on https://github.com/coreos/coreos-assembler/pull/2876, but only once.

https://storage.googleapis.com/origin-ci-test/pr-logs/pull/coreos_coreos-assembler/2876/pull-ci-coreos-coreos-assembler-rhcos-4.10-rhcos/1529464945303883776/build-log.txt

Just noting here in case we see it again.

Start with the rhcos.upgrade.from-ocp-rhcos test and the panic stack in kola/tests/rhcos/upgrade.go, especially downloadLatestReleasedRHCOS at line 376 and its caller at line 212. Reproduce the RHCOS 4.9-to-4.10 path in Prow and inspect the linked build log. Done means the test completes without the nil pointer panic.