upspin/proto: refdata may be nil
upspin/proto: refdata may be nil

It happens both ways. In Put, if a non-RPC error is not nil,
refdata can still be nil and we crash on store/remote/remote.go:73.

In Get, the API says refdata may be nil if we're returning the bytes
directly. This does happen in practice when we do store.Put followed
immediately by a store.Get faster than GCP can upload to GCS.

Change-Id: I2850ae33afe64ba8804a662d3463fb94a495fc2f
Reviewed-on: https://upspin-review.googlesource.com/6063
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@google.com>

upspin/upspin,upspin/upspin