crypto/elliptic: inline marshaling into nistec pointFromAffine

crypto/elliptic: inline marshaling into nistec pointFromAffine

Marshal behavior for invalid points is undefined, so don't use it to
check if points are valid.

For #52182

Change-Id: If167893bc4b029f71bb2528564f2bd96bee7221c
Reviewed-on: https://go-review.googlesource.com/c/go/+/382994
Run-TryBot: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>