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 SIGSEGV dockerd 20.10.2 docker/for-linux#1186 for which fifo.Close(): prevent possible panic if fifo is nil #32 was added as a workaround.