nested call_priority seg faults when compiled (#902)

[eclipse 12]: [user].
 f :-
 call_priority((call_priority(writeln(lower),10),writeln(higher)), 5).
 Segmentation fault

and 

Version 5.1.0, Thu Jan 11 19:49 2001
[eclipse 1]: [user].
 f :-
 call_priority((call_priority(false,11) -> writeln(thenbranch) ;
    writeln(elsebranch)), 10) .
 Segmentation fault