The existing implementation returns nil, ErrBufferFull when n > len(b.buf),
now it will return any data in the buffer and ErrBufferFull.

Fixes #14121

Change-Id: Ie52d32ccd80e4078ebfae6e75393c89675959ead
Reviewed-on: https://go-review.googlesource.com/19091
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>