math/big: fix spurious race in Rat.Denom, Float.SetRat

Rat maintains the invariant that x.b.neg is always false,
but Rat.Denom was writing x.b.neg = false itself too.
That makes Rat.Denom a writing operation, when it should
be a read-only operation. That in turn makes it unsafe to
use from multiple goroutines, which is highly unexpected.
Make it read-only and therefore race-free again.

Fixes #50473.

Change-Id: I97b87913954511e5200c0665d16b9ed63422e505
Reviewed-on: https://go-review.googlesource.com/c/go/+/375935


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>