sign-ed25519: Convert functions to new style
sign-ed25519: Convert functions to new style

The "new style" code generally avoids `goto err` because it conflicts
with `__attribute__((cleanup))`.  This fixes a compiler warning.

Signed-off-by: Denis Pynkin <3fbf95ee68899518ee8799d35f88c8f7982aa936@collabora.com>

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree