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 <4f40cda291c5f9634e1affd3db44947af61f705c@golang.org>
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>
Reviewed-by: Roland Shoemaker <0d270388f2f92757a5de0f4bd891d3b392c44c4f@golang.org>

golang/go,golang/go,golang/go,nawawi/go,nawawi/go,golang/go,nawawi/go,nawawi/go,nawawi/go,golang/go,nawawi/go,golang/go,golang/go,nawawi/go,golang/go,nawawi/go