Fix nil pointer dereference

[ebfe]

I triggered this with:

$ scw wait abcdef
panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x0 pc=0x52137e]

goroutine 1 [running]:
github.com/scaleway/scaleway-cli/pkg/api.WaitForServerState(0xc820086580, 0xc8203e4c90, 0x24, 0x8993b8, 0x7, 0x0, 0x0, 0x0)
        go/src/github.com/scaleway/scaleway-cli/pkg/api/helpers.go:392 +0x30e
github.com/scaleway/scaleway-cli/pkg/api.WaitForServerStopped(0xc820086580, 0xc8203e4c90, 0x24, 0xc8203e4c90, 0x0, 0x0)
        go/src/github.com/scaleway/scaleway-cli/pkg/api/helpers.go:468 +0x5a
github.com/scaleway/scaleway-cli/pkg/commands.RunWait(0x7f6afb1eb220, 0xc82002a008, 0x7f6afb1eb308, 0xc82002a010, 0x7f6afb1eb308, 0xc82002a018, 0xc82000cc80, 0x28, 0x28, 0xc82000a2f0, ...)
        go/src/github.com/scaleway/scaleway-cli/pkg/commands/wait.go:25 +0x122
github.com/scaleway/scaleway-cli/pkg/cli.runWait(0xab8220, 0xc82000a2f0, 0x1, 0x1, 0x0, 0x0)
        go/src/github.com/scaleway/scaleway-cli/pkg/cli/cmd_wait.go:35 +0x13c
github.com/scaleway/scaleway-cli/pkg/cli.Start(0xc82000a2e0, 0x2, 0x2, 0xc82000bc80, 0x0, 0x0, 0x0)
        go/src/github.com/scaleway/scaleway-cli/pkg/cli/main.go:113 +0xc76
main.main()
        go/src/github.com/scaleway/scaleway-cli/cmd/scw/main.go:16 +0x8e
...

But it's not easy to reproduce.

[QuentinPerez]

Good catch :) LGTM

[moul]

LGTM