Fix crashes in keychain Get & Remove

[dgoodlad]

These lines were causing serviceRef to be released twice. It looks like some copy/paste shenanigans from the blocks just above where I've made these changes, e.g. keychain.go:59. I think it's pretty safe to assume from the context that these lines were meant to release accountRef not serviceRef.

These were causing a crash on macOS 10.12 beta that looked like:

SIGTRAP: trace trap
PC=0x7fff86a8b985 m=5
signal arrived during cgo execution

goroutine 1 [syscall, locked to thread]:
runtime.cgocall(0x442a000, 0xc8200dfe20, 0xc800000000)
        /usr/local/Cellar/go/1.5.3/libexec/src/runtime/cgocall.go:120 +0x11b fp=0xc8200dfdf0 sp=0xc8200dfdc0
github.com/99designs/aws-vault/keyring._Cfunc_CFRelease(0x4f00000)
        ??:0 +0x31 fp=0xc8200dfe20 sp=0xc8200dfdf0
github.com/99designs/aws-vault/keyring.(*keychain).Get(0xc8200e5380, 0x7fff5fbffa77, 0xa, 0x7fff5fbffa77, 0xa, 0xc820017050, 0x87, 0x87, 0xc820
013800, 0x16, ...)
        /Users/lachlan/Projects/99designs/go/src/github.com/99designs/aws-vault/keyring/keychain.go:117 +0x97c fp=0xc8200dffb8 sp=0xc8200dfe20
main.(*KeyringProvider).Retrieve(0xc820013740, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)

[lox]

Ugh. Thanks, good catch. Should really prioritize #58. I'm waiting for upstream to merge it, but perhaps we should just use https://github.com/99designs/go-keychain.

[dgoodlad]

@lox yeah there's a lot of work that seems to happen in this repo to support the keychain module and bugs therein; I did follow #58 through to keybase/go-keychain#4 - reckon it just needs another ping and review there...