The combination of two prior CLs can cause panics:
* CL/17873: make chained multiReader Read more efficient
* CL/28533: make MultiReader nil exhausted Readers for earlier GC

The first CL allows MultiReader to "inherit" another MultiReader's list of Readers
for efficiency reasons. This is problematic when combined with the
later CL since that can set prior Readers in that list to nil for GC reasons.
This causes panics when two MultiReaders are used together (even synchronously).

To fix this, rather than setting consumed Readers as nil, we set them with
a special eofReader that always returns EOF.

Fixes #18232

Change-Id: I2a9357ab217e3d54d38ea9022d18e4d14f4182d3
Reviewed-on: https://go-review.googlesource.com/34140Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>