This removes a sharp corner in the API where |ECDH_compute_key| assumed
that callers were either using ephemeral keys, or else had already
checked that the public key was on the curve.
A public key that's not on the curve can be in a small subgroup and thus
the result can leak information about the private key.
This change causes |EC_POINT_set_affine_coordinates_GFp| to require that
points are on the curve. |EC_POINT_oct2point| already does this.
Change-Id: I77d10ce117b6efd87ebb4a631be3a9630f5e6636
Reviewed-on: https://boringssl-review.googlesource.com/5861
Reviewed-by: Adam Langley <agl@google.com>
38feb990a1