[All modules] NULL checks do not work at all cases.

[mfragkoulis]

e.g. multiple arguments to functions used in access paths.
Pointers to character arrays are checked but they are
not first class citizens of the NULL checking mechanism.
The mechanism requires refactoring.

[mfragkoulis]

Include NULL checks in USING LOOP directives,
e.g. list_for_each_entry(tuple_iter,&any_dstr->partial,lru)

[mfragkoulis]

Inherent checks for loops are not sufficient sometimes, e.g.
iterating super blocks requires sth like the following:
if (hlist_unhashed(&sb->s_instances))
continue;
GIve the designer the ability to define such additional conditions through a directive in the PiCO QL DSL.