compress/zlib: detect truncated streams

compress/zlib: detect truncated streams

Reader failed to detect truncated streams since calls to
io.ReadFull did not check if the error is io.EOF.

Change-Id: I86c497519daaaccefc6eb5617ddcd8fd3b99f51b
Reviewed-on: https://go-review.googlesource.com/14835
Reviewed-by: Nigel Tao <nigeltao@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>