bsc1200022: fifo.Close(): prevent possible panic if fifo is nil

I'm not sure if this is the right approach, and synchronisation should probably
be added elsewhere to fix the underlying issue.

Trying to prevent a panic that was seen on container restore in th docker daemon:

    panic: runtime error: invalid memory address or nil pointer dereference
    [signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0x5586c892a7a4]

    goroutine 420 [running]:
    github.com/docker/docker/vendor/github.com/containerd/fifo.(*fifo).Close(0x0, 0x0, 0x0)
            /go/src/github.com/docker/docker/vendor/github.com/containerd/fifo/fifo.go:208 +0x44
    github.com/docker/docker/vendor/github.com/containerd/containerd/cio.(*cio).Close(0xc000d06f60, 0x5586cb5654d0, 0xc000d8e9e8)
            /go/src/github.com/docker/docker/vendor/github.com/containerd/containerd/cio/io.go:203 +0x90
    github.com/docker/docker/libcontainerd/remote.(*client).Restore.func1(0xc0008bf820, 0xc0008a2040)
            /go/src/github.com/docker/docker/libcontainerd/remote/client.go:86 +0x5a
    github.com/docker/docker/libcontainerd/remote.(*client).Restore(0xc00098e5b0, 0x5586cb61c7c0, 0xc000052088, 0xc0011b6500, 0x40, 0xc0008bf810, 0x5586cb05cf00, 0xffffffffffffffff, 0x0, 0x0, ...)
            /go/src/github.com/docker/docker/libcontainerd/remote/client.go:107 +0x923
    github.com/docker/docker/daemon.(*Daemon).restore.func3(0xc00079d9e0, 0xc000a38230, 0xc00000c1e0, 0xc00079d9a8, 0xc000d84f00, 0xc000d84ed0, 0xc000d84ea0, 0xc00128a280)
            /go/src/github.com/docker/docker/daemon/daemon.go:351 +0x48a
    created by github.com/docker/docker/daemon.(*Daemon).restore
            /go/src/github.com/docker/docker/daemon/daemon.go:319 +0x4b3

If the fifo is nil, there's nothing to be done in Close(), so returning early
in that situation.

Backport: <containerd/fifo#32>
SUSE-Bugs: bsc#1200022
Signed-off-by: Sebastiaan van Stijn <[email protected]>