dh: fix libcrypto dh_ctx leak in ssh_dh_cleanup
Ensure to free the dh_ctx member in ssh_dh_cleanup to match
the allocation in ssh_dh_init_common.
The before-and-after of this change can be observed with the pkd
tests and valgrind:
valgrind --leak-check=full \ ./pkd_hello -i1 -t torture_pkd_openssh_dsa_rsa_diffie_hellman_group16_sha512
Signed-off-by: Jon Simons <firstname.lastname@example.org>
Reviewed-by: Andreas Schneider <email@example.com>