Return nil when openFifo returns nil

[corhere]

The return statement in the OpenFifo wrapper function implicitly boxes the *fifo pointer into an io.ReadWriteCloser interface value, even when the value is (*fifo)(nil). It is the same gotcha described in https://go.dev/doc/faq#nil_error and causes the same sorts of confusing situations. Modify the OpenFifo wrapper function to return a nil interface value when openFifo returns a nil pointer value.

• This appears to have been the only code path by which a (*fifo)(nil) value could escape the package, and therefore the root cause of https://github.com/docker/for-linux/issues/1186 for which #32 was added as a workaround.

[corhere]

@cpuguy83 @thaJeztah @AkihiroSuda @dmcgowan PTAL

[thaJeztah]

Oh! Saw the PR title and had to think of https://github.com/containerd/fifo/pull/32, but I see you linked that above, nice 👍