auth: Fix simpleToken to respect disabled state for assign

[jpbetz]

TokenProviders can be enabled and disabled. The tokenSimple implementation is disabled when the simpleTokenKeeper field is set tonil.

func (t *tokenSimple) assignSimpleTokenToUser(...), however, is missing a t.simpleTokenKeeper != nil check like those found in (t *tokenSimple) invalidateUser(...) and (t *tokenSimple) info(...), and if it is called when tokenSimple is disabled, a nil pointer derference panic will occur. This appears to be the cause of #8608:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x90197c]

goroutine 133 [running]:
github.com/coreos/etcd/cmd/vendor/github.com/coreos/etcd/auth.(*simpleTokenTTLKeeper).addSimpleToken(0x0, 0xc4239045c0, 0x16)
	/root/etcd/gopath/src/github.com/coreos/etcd/cmd/vendor/github.com/coreos/etcd/auth/simple_token.go:59 +0x3c
github.com/coreos/etcd/cmd/vendor/github.com/coreos/etcd/auth.(*tokenSimple).assignSimpleTokenToUser(0xc4203f64e0, 0xc421688178, 0x4, 0xc4239045c0, 0x16)
	/root/etcd/gopath/src/github.com/coreos/etcd/cmd/vendor/github.com/coreos/etcd/auth/simple_token.go:128 +0x12d
github.com/coreos/etcd/cmd/vendor/github.com/coreos/etcd/auth.(*tokenSimple).assign(0xc4203f64e0, 0x14db740, 0xc4238feae0, 0xc421688178, 0x4, 0xbb, 0x7ffd9f077a28, 0x0, 0x0, 0xc42002e700)
	/root/etcd/gopath/src/github.com/coreos/etcd/cmd/vendor/github.com/coreos/etcd/auth/simple_token.go:191 +0x1e7
...

Adding the check appears safe and is consistent with the intended behavior of the disabled state.

[gyuho]

s/2016/2017/?

[gyuho]

Also staticcheck found

Checking staticcheck...
auth/simple_token_test.go:63:2: this value of authInfo is never used (SA4006)

https://travis-ci.org/coreos/etcd/jobs/287753236#L1216

Could you remove authInfo? Thanks.

[xiang90]

/cc @mitake

[mitake]

@jpbetz lgtm, thanks a lot for fixing this!

[mitake]

@gyuho @xiang90 I'm merging this PR because the CI failures wouldn't be related to the change. Please let me know if there are problems.