For all operations that can be applied on file descriptors, we have
special treatment to permit the File to be nil. In that case, operations
are called against the Node object instead.

This check currently seems missing from SetAttr(), causing invocations
of the truncate(1) command line tool to cause a crash:

        panic: runtime error: invalid memory address or nil pointer dereference
        [signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0xa30d0d]

        goroutine 12 [running]:
        github.com/hanwen/go-fuse/fuse/nodefs.(*rawBridge).SetAttr(0xc00013ecc0, 0xc0003a8000, 0xc0003ba0f0, 0xa27b98)
                external/com_github_hanwen_go_fuse/fuse/nodefs/fsops.go:203 +0x58d
        github.com/hanwen/go-fuse/fuse.doSetattr(0xc0003a4000, 0xc0003ba000)
                external/com_github_hanwen_go_fuse/fuse/opcode.go:171 +0x64
        github.com/hanwen/go-fuse/fuse.(*Server).handleRequest(0xc0003a4000, 0xc0003ba000, 0xc0003ba000)
                external/com_github_hanwen_go_fuse/fuse/server.go:431 +0x26b
        github.com/hanwen/go-fuse/fuse.(*Server).loop(0xc0003a4000, 0xc00000b101)
                external/com_github_hanwen_go_fuse/fuse/server.go:403 +0x18f
        created by github.com/hanwen/go-fuse/fuse.(*Server).readRequest
                external/com_github_hanwen_go_fuse/fuse/server.go:291 +0x2d8