Hold on a minute!
I've just applied that patch to the latest mesa - now correct me if I'm wrong -
but the source already has that change applied:
https://gitlab.freedesktop.org/mesa/mesa/blob/master/src/gallium/auxiliary/cso_cache/cso_cache.c

>cso_insert_state(struct cso_cache *sc,
>                unsigned hash_key, enum cso_cache_type type,
>                 void *state)
>{
>   struct cso_hash *hash = _cso_hash_for_type(sc, type);
>   sanitize_hash(sc, hash, type, sc->max_size);
>
>   return cso_hash_insert(hash, hash_key, state);
>}

So how can it work perfectly for one hour or more then crash and burn on two
other occasions. I feel a bit daft now too =(

I'll have to clear out everything, cache, mesa drivers in /opt/mesa, re-compile
the latest and test it for even longer.