Fix bug in nistp224/256/521 where have_precompute_mult always returns 0

During precomputation if the group given is well known then we memcpy a
well known precomputation. However we go the wrong label in the code and
don't store the data properly. Consequently if we call have_precompute_mult
the data isn't there and we return 0.

RT#3600

Reviewed-by: Richard Levitte <levitte@openssl.org>