Fix ECIES params nil bug

* Change ECIES params init function to static var as it does not have
  state; fixes TestMarshalencryption.

Correct ECIES shared key length check

* Ensure the ECIES shared key is padded with zero bytes if it's
  smaller than the requested key length.
* Split the ECIES shared key error into two;
  one for when the generated key is too big for the params
  and one for when it's nil (point of infinity returned by the
  curve scalar multiplication).

Update reference pdf links