formatFloat should restore the original f.wid value before
returning. Callers should not have to save and restore f.wid.

Fixes: #14642

Change-Id: I531dae15c7997fe8909e2ad1ef7c376654afb030
Reviewed-on: https://go-review.googlesource.com/20179
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>