fmt: properly handle early io.EOF Reads in readRune.readByte

Change https://golang.org/cl/19895 caused a regression
where the last character in a string would be dropped if it was
accompanied by an io.EOF.

This change fixes the logic so that the last byte is still returned
without a problem.

Fixes #16393

Change-Id: I7a4d0abf761c2c15454136a79e065fe002d736ea
Reviewed-on: https://go-review.googlesource.com/24981


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>