Do not panic when we encounter nil interface values which are
invalid values for gob. Previously this wasn't caught yet
we were calling reflect.*.Type() on reflect.Invalid values
thereby causing panic:
  `panic: reflect: call of reflect.Value.Type on zero Value.`
which is a panic not enforced by encoding/gob itself.
We can catch this and send back an error to the caller.

Fixes #16204

Change-Id: Ie646796db297759a74a02eee5267713adbe0c3a0
Reviewed-on: https://go-review.googlesource.com/24989Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Rob Pike <r@golang.org>