Fixed error introduced in commit f2be92b94dad3c6cbdf79d99a324804094cf1617
that fixed PR#3450 where an existing cast masked an issue when i was changed
from int to long in that commit

Picked up on z/linux (s390) where sizeof(int)!=sizeof(long)

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
crypto/ecp_nistz256.c: harmonize error codes.

Reviewed-by: Dr. Stephen Henson <steve@openssl.org>