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.