Fix 'vault auth' panic (#5473)
Fix 'vault auth' panic (#5473)

Running 'vault auth' with no parameters was panicking:

panic: assignment to entry in nil map
	github.com/hashicorp/vault/command/login.go:255 +0xdee

Now it will show help.
joelthompson/vault,rnaveiras/vault,hashicorp/vault,rnaveiras/vault,quixoten/vault,kingland/vault,kingland/vault,hartsock/vault,quixoten/vault,hashicorp/vault,quixoten/vault,kingland/vault,hartsock/vault,hashicorp/vault,kingland/vault,joelthompson/vault,rnaveiras/vault,rnaveiras/vault,quixoten/vault,hartsock/vault,hartsock/vault,hartsock/vault,kingland/vault,joelthompson/vault,rnaveiras/vault,hashicorp/vault,joelthompson/vault,hashicorp/vault,hashicorp/vault