client: handle pathological nil entry for link
client: handle pathological nil entry for link

Need some fancy scaffolding to test, so this is untested.
We might one day create such a thing, a way to program a
misbehaving server for package-level tests.

But not now.

Fixes #451.

Change-Id: Ib83ac2785921342dfb912af2f150eef7c3ee4b9e
Reviewed-on: https://upspin-review.googlesource.com/12320
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

upspin/upspin,upspin/upspin