Fix to Issue #84

We can't pass a constant zero as a divisor when building KLEE expressions,
because KLEE invokes llvm::APInt's div functions, which crash on zero value
divisors.

To fix this, we check if the Divisor is a zero constant. If it is, then
we record this UB and simply return a KLEE constant zero rather than going
through KLEE's api(s) such as SDivExpr::create.