ecc: fix memory leaks.

cipher/ecc.c (ecc_generate): Fix memory leak on error of
_gcry_pk_util_parse_flaglist and _gcry_ecc_eddsa_encodepoint.
(ecc_check_secret_key): Fix memory leak on error of
_gcry_ecc_update_curve_param.
(ecc_sign, ecc_verify, ecc_encrypt_raw, ecc_decrypt_raw): Remove
unnecessary sexp_release and fix memory leak on error of
_gcry_ecc_fill_in_curve.
(ecc_decrypt_raw): Fix double free of the point kG and memory leak
on error of _gcry_ecc_os2ec.