gost_ec_keyx: Fix CID 253282 Dereference null return value

`param' is used unconditionally in pkey_GOST_ECcp_encrypt() so we can
unconditionally check (vs in any particular `if' branch) for it's being
non-NULL. This is similar to how pkey_GOST_ECcp_decrypt() works with
`param'.

Closes #174.