math/big: call norm when returning success from Rat SetString
After CL 24430, reflect.DeepEqual no longer returns true when comparing
a *Rat built with (*Rat).SetString("0") with one built with
(*Rat).SetInt64(0).
These should be equivalent, but because (*Rat).SetString does not call
norm() when returning the zero value, the result of reflect.DeepEqual
will be false.
One could suggest that developers should use (*Rat).Cmp instead
of relying on reflect.DeepEqual, but if a (*Rat) is part of a
larger struct that is being compared, this can be cumbersome.
This is fixed by calling z.norm() when returning zero in SetString.
Fixes #50944
Change-Id: Ib84ae975bf82fe02d1203aa9668a01960c0fd59d
Reviewed-on: https://go-review.googlesource.com/c/go/+/364434 Reviewed-by: Katie Hockman <katie@golang.org>
Trust: Katie Hockman <katie@golang.org>
Trust: Ian Lance Taylor <iant@golang.org>