Ensure that all errors (including io.EOF) are persistent across method
calls on zlib.Reader. Furthermore, ensure that these persistent errors
are properly cleared when Reset is called.

Fixes #14675

Change-Id: I15a20c7e25dc38219e7e0ff255d1ba775a86bb47
Reviewed-on: https://go-review.googlesource.com/20292Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>