Simplify ec_group_new_from_data() further

Simplify ec_group_new_from_data() further

We have a BN_CTX available, so we may as well use it. This simplifies
the cleanup path at the cost of a bit more code in the setup. Also use
an extra BIGNUM for the cofactor. Reusing x for this is just silly. If
you were really going to avoid extra allocations, this entire function
could easily have been written with three BIGNUMs.

ok jsing