[crypto/ec] don't assume points are of order group->order

(cherry picked from commit 01fd5df77d401c87f926552ec24c0a09e5735006)

Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6549)