compress/gzip: fix error handling in Read

compress/gzip: fix error handling in Read

The Read logic should not assume that only (0, io.EOF) is returned
instead of (n, io.EOF) where n is positive.

The fix done here is very similar to the fix to compress/zlib
in CL/20292.

Change-Id: Icb76258cdcf8cfa386a60bab330fefde46fc071d
Reviewed-on: https://go-review.googlesource.com/21308
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>