Fix handling of points at infinity in ec_GFp_simple_points_make_affine.
When inverting an array of Z coordinates, the algorithm is supposed to
treat any 0 essentially like a 1 to remain in the multiplicative group;
however, for one of the cases, we incorrectly multiplied by 0 and thus
ended up with garbage.
This change saves several EC routines from crashing when an EC_KEY is
missing a public key. The public key is optional in the EC private key
format and, without this patch, running the following through `openssl
ec` causes a crash:
-----BEGIN EC PRIVATE KEY-----
MBkCAQEECAECAwQFBgcIoAoGCCqGSM49AwEH
-----END EC PRIVATE KEY-----
Initial fork from f2d678e6e89b6508147086610e985d4e8416e867 (1.0.2 beta).
(This change contains substantial changes from the original and
effectively starts a new history.)
