message: teach int2ptr(non-zero) != null
  this allow us to verify a few more gcc functions
diff: @@ -1718,9 +1718,13 @@ expr Memory::ptr2int(const expr &ptr) const {
  expr Memory::int2ptr(const expr &val) const {
  assert(!memory_unused());
  // TODO
  + // Approximation:
  + // int2ptr(0) => null
  + // else => non-null
  expr null = Pointer::mkNullPointer(*this).release();
  expr fn = expr::mkUF("int2ptr", { val }, null);
  state->doesApproximation("inttoptr", fn);
  + state->addPre(fn != null);
  return expr::mkIf(val == 0, null, fn);
  }
  