fix for #if handling: Don't eval the control expression of the first #if of a block when already in a false block; it might not be valid. Reported by Baptiste Daroussin.

1.
;
2.
(a, );
(, b);
( , );
(a,);
(,b);
(,);
3.
(a, b, );
(a, , c);
(, , c);
(a, , );
(, b, );
(, , c);
(, , )
(a,b,);
(a,,c);
(,,c);
(a,,);
(,b,);
(,,c);
(,,)