Fix leak of secrecy in ecdh_compute_key()

A temporary buffer containing g^xy was not cleared in ecdh_compute_key()
before freeing it, so the shared secret was leaked in memory.

Reviewed-by: Kurt Roeckx <kurt@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(cherry picked from commit 0e469016)