Added test case for a segfaulting RESP file.
Avoid segfault by declaring a variable to be volatile.

The problem was that clang (not tested with gcc) somehow messed with
local variables when using setjmp/longjump when optimization was turned
on.

The tip to turn it into a volatile variable is from here although is
does not appear to be the exactly the same problem:
http://stackoverflow.com/a/7996969
Raise a more specific error when evalresp fails to calculate a response.
Adjust test case to catch exception.
Merge pull request #819 from krischer/evalresp_segfaults

Remove a segfault in evalresp