bufio: Reader.Read may return io.EOF even when it has data buffered

bufio: Reader.Read may return io.EOF even when it has data buffered

If reading 0 bytes, don't return the error from the underlying
io.Reader if there is still data buffered.

Fixes #32693

Change-Id: I12a97bd6003c638c15d41028942f27edf88340e2
Reviewed-on: https://go-review.googlesource.com/c/go/+/182997
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>