fs/fuse: don't dereference fuse.DeviceFD.fs if it is nil
fs/fuse: don't dereference fuse.DeviceFD.fs if it is nil

PiperOrigin-RevId: 342992936

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor