keycodes: don't try to copy zero key aliases

Move the aliases copy to within the (num_key_aliases > 0) block.

Passing info->aliases into this fuction with invalid aliases will
cause log messages but num_key_aliases stays on 0. The key_aliases array
is never allocated and remains NULL. We then loop through the aliases, causing
a null-pointer dereference.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>