Panics before calling az acr login

[derekperkins]

I am using https://github.com/GoogleContainerTools/skaffold to deploy to an AKS cluster, but it's annoying to have to re-run az acr login every hour or so. Someone suggested to use this credential helper (https://github.com/GoogleContainerTools/skaffold/issues/550), and after installing but before running az acr login to initialize the helper, skaffold run tried to use the helper, causing a panic. After running az acr login, the deployment worked, but as I was already logged in, I don't think that the credential helper executed.

I'll report back after my creds timeout and it tries to use the helper again.

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x38 pc=0x12d8f69]

goroutine 1 [running]:
main.receiveChallengeFromLoginServer(0xc4202f05a0, 0x19, 0x0, 0x0, 0x0)
	/build-root/src/docker-credential-acr/acr_login.go:90 +0x149
main.GetUsernamePassword(0xc4202f05a0, 0x19, 0xc42031e000, 0x3a7, 0x1, 0xc42031e000, 0x3a7, 0x0, 0x0, 0xc420183c48)
	/build-root/src/docker-credential-acr/acr_login.go:70 +0x101
main.(*storeWrapper).Get(0xc42000c470, 0xc4202f05a0, 0x19, 0x19, 0xc4202f05a0, 0x19, 0x0, 0x10000c420183ce0, 0x0)
	/build-root/src/docker-credential-acr/program.go:58 +0x143
docker-credential-acr/vendor/github.com/docker/docker-credential-helpers/credentials.Get(0x1518240, 0xc42000c470, 0x1513540, 0xc42000c010, 0x1513580, 0xc42000c018, 0x8, 0x1342860)
	/build-root/src/docker-credential-acr/vendor/github.com/docker/docker-credential-helpers/credentials/credentials.go:104 +0x239
docker-credential-acr/vendor/github.com/docker/docker-credential-helpers/credentials.HandleCommand(0x1518240, 0xc42000c470, 0x7ffeefbffa8d, 0x3, 0x1513540, 0xc42000c010, 0x1513580, 0xc42000c018, 0xc420183ef8, 0x100f1c8)
	/build-root/src/docker-credential-acr/vendor/github.com/docker/docker-credential-helpers/credentials/credentials.go:56 +0x1dd
docker-credential-acr/vendor/github.com/docker/docker-credential-helpers/credentials.Serve(0x1518240, 0xc42000c470)
	/build-root/src/docker-credential-acr/vendor/github.com/docker/docker-credential-helpers/credentials/credentials.go:41 +0x1cb
main.main()
	/build-root/src/docker-credential-acr/program.go:186 +0xfb