Remove a segfault in evalresp

This PR removes a segfault in evalresp with certain input files.

The problem was that clang (not tested with gcc) messes with a variable when using `setjmp`/`longjmp` (evalresp likes to jump around...) when optimization was turned on. Setting the variable to `volatile` fixes this. This variable is only accessed a couple time per run so performance is not affected.

It still does not compute the response for the faulty file but it now raises an exception and does not segfault any longer.