bufio: do not cache Read errors

bufio: do not cache Read errors

Reader previously had cached an error from the underlying reader
and would return it on every subsequent call to Read.  The Reader
will now return the error only once, and subsequent calls will result
in a new Read call to the underlying Reader.

Fixes #1934.

R=bradfitz, rogpeppe, rsc
CC=golang-dev
https://golang.org/cl/4528133