fixed core dump on division by zero, now it gives an error in lisp

division by zero is only an issue for integers 1.0/0.0 == inf but
1/0 == floating-point excepiton (core-dump). So I added a quick check to
the primitive integer division function and now no more core dumps. I
also added a test for this in my tests (i.e (assert(errorp (/ 1 0))))