Improve undef checks

[lou1306]

The semantics of our guards and link predicates rely on the assumption that the examined variables have a defined value. That is, the process

x > 3 -> P

should only proceed as P if the variable x has a value that is higher than 3.

The current implementation stores 0x7FFFFFFF in all variables that are initialized to undef. This should be treated as a sentinel value everywhere a guard or link predicate is evaluated and it should disallow the evaluation to proceed.

By the way, this also means that the logic of guards is "intuitionistic", in the sense that guards like this:

φ or !φ -> P

are not guaranteed to succeed.