SSH command panic when using CA mode with host-key-mount-point

[rmbrad]

When using the SSH command in CA mode if host-key-mount-point flag is used the client panics with runtime error: invalid memory address or nil pointer dereference.

Environment:

• Vault Version: Vault v0.9.3 ('7d73ac4c967b3b8726c71e00c017129e1d36ad3c')
• Operating System/Architecture: CentOS7/x86_64

Expected Behavior:
Establishment of a SSH connection to host without prompt to verify identity of host.

Actual Behavior:
The command panics:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x7b01e3]

goroutine 1 [running]:
github.com/hashicorp/vault/api.(*Client).NewRequest(0x0, 0x22523cc, 0x3, 0xc42041af60, 0x19, 0x0)
	/go/src/github.com/hashicorp/vault/api/client.go:472 +0x43
github.com/hashicorp/vault/api.(*Logical).Read(0xc420553a10, 0xc420553958, 0x15, 0x0, 0x0, 0x0)
	/go/src/github.com/hashicorp/vault/api/logical.go:47 +0xbf
github.com/hashicorp/vault/command.(*SSHCommand).handleTypeCA(0xc420534500, 0x7fff73d29637, 0x9, 0xc4203bfb20, 0x9, 0xc420553c48, 0x0, 0x0, 0x0)
	/go/src/github.com/hashicorp/vault/command/ssh.go:394 +0x1241
github.com/hashicorp/vault/command.(*SSHCommand).Run(0xc420534500, 0xc42000e0d0, 0x1, 0x1, 0xc420538250)
	/go/src/github.com/hashicorp/vault/command/ssh.go:313 +0x5d3
github.com/hashicorp/vault/vendor/github.com/mitchellh/cli.(*CLI).Run(0xc4204dba40, 0xc420148d20, 0xc420506940, 0xc420148d20)
	/go/src/github.com/hashicorp/vault/vendor/github.com/mitchellh/cli/cli.go:255 +0x1eb
github.com/hashicorp/vault/command.Run(0xc42000e080, 0x6, 0x6, 0x1b38259)
	/go/src/github.com/hashicorp/vault/command/main.go:47 +0x411
main.main()
	/go/src/github.com/hashicorp/vault/main.go:10 +0x63

Steps to Reproduce:
Execute:

vault ssh -mode=ca -mount-point=<ssh-client-mount> -role=<valid-role> -host-key-mount-point=<ssh-host-mount> <user>@<host>

[vishalnayak]

Fixed by #3909.