variables: voidify callback addition functions
In case of memory error, the functions will abort (table macros). Even if it did not, callback deletion would later abort failing to find the callback instead. (The API could be redesigned not to require memory allocation if necessary.) In case of nonexistent variable, log an error and fail safe as before. This lead to deadlocks of misbehaviours as before - no changes here.
Showing with 10 additions and 12 deletions