ecc: Store EdDSA session key in secure memory.

* cipher/ecc-eddsa.c (_gcry_ecc_eddsa_sign): use mpi_snew to allocate
session key.

An attacker who learns the EdDSA session key from side-channel
observation during the signing process, can easily revover the long-
term secret key. Storing the session key in secure memory ensures that
constant time point operations are used in the MPI library.

• Signed-off-by: Jo Van Bulck <jo.vanbulck@cs.kuleuven.be>