Fix ed25519 ignoring the public key offset

- Thanks to https://github.com/TimoRoth

Author: peterdettman

crypto/src/math/ec/rfc8032/Ed25519.cs

@@ -330,7 +330,7 @@ private static void ImplSign(IDigest d, byte[] h, byte[] s, byte[] pk, int pkOff
 
             Dom2(d, phflag, ctx);
             d.BlockUpdate(R, 0, PointBytes);
-            d.BlockUpdate(pk, 0, PointBytes);
+            d.BlockUpdate(pk, pkOff, PointBytes);
             d.BlockUpdate(m, mOff, mLen);
             d.DoFinal(h, 0);