fix calculator alarm after result

The alarm function (used to guard against run-away evals) needs to be
canceled after it's been initiated or it will deliver a SIGALRM to the
process after the result has been returned [shutting down duckpan].

I looked into this after realizing that queries for `1+1` were failing in `duckpan query` strangely after delivering a correct result. I'm not quite sure why this isn't failing in production, maybe it's got a different signal handler in its context, maybe it's getting restarted, or maybe this is only a problem on my system :-).

[perldoc alarm](http://perldoc.perl.org/functions/alarm.html)