Fix panic when `/dev/null` does not exist

[bduffany]

Fixes an issue that the init() func in splice.go panics if /dev is not yet mounted.

This allows supporting go-based init binaries which set up all the filesystems on the machine, including both /dev as well as FUSE-mounted dirs.

[hanwen]

if possible, I'd prefer a gerrithub review.

[bduffany]

if possible, I'd prefer a gerrithub review.

Sorry, I am not comfortable with the permissions that Gerrithub is requesting from my GitHub account, and I don't see a way to log in without a GitHub link :confused:

[rfjakob]

This has already been merged as https://github.com/hanwen/go-fuse/commit/b5c46276a19e02b7912f998e4ccee85330458f4d