Refactor: Use if for two-way branching; Compare against 0 explicitly

Refactor: Use if for two-way branching; Compare against 0 explicitly

I think that the use of switch for the two-way branches could have
been a performance optimization. I think it's clearer to use if,
especially in the fall-through case.

As setjmp() is known to return with either 0 or something else, we
should compare against 0 and not against some define (for which I
again and again wonder if it stands for 0 or for 1).