Check key_len in secret_to_key_new()

This bug shouldn't be reachable so long as secret_to_key_len and
secret_to_key_make_specifier stay in sync, but we might screw up
someday.

Found by coverity; this is CID 1241500
Comment-out dead code in ed25519/ref10

There are some loops of the form

       for (i=1;i<1;++i) ...

And of course, if the loop index is initialized to 1, it will never
be less than 1, and the loop body will never be executed.  This
upsets coverity.

Patch fixes CID 1221543 and 1221542