Correct checks for ecc secret key

* cipher/ecc.c (check_secret_key): replace wrong comparison of Q and
sk->Q points with correct one.

Currently check_secret_keys compares pointers to coordinates of Q
(calculated) and sk->Q (provided) points. Instead it should convert them
to affine representations and use mpi_cmp to compare coordinates.

This has an implication that keys that were (erroneously) verified as
valid could now become invalid.

• Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>