message: change malloc(poison) to yield poison instead of UB
  there's no need to be that strong
diff: @@ -1651,7 +1651,6 @@ void Malloc::print(std::ostream &os) const {
  StateValue Malloc::toSMT(State &s) const {
  auto &[sz, np] = s[*size];
  - s.addUB(np);
  // TODO: malloc's alignment is implementation defined.
  auto p = s.getMemory().alloc(sz, 8, Memory::HEAP);
  @@ -1661,7 +1660,7 @@ StateValue Malloc::toSMT(State &s) const {
  auto nullp = Pointer::mkNullPointer(s.getMemory());
  auto flag = expr::mkFreshVar("malloc_isnull", expr(true));
  s.addQuantVar(flag);
  - return { expr::mkIf(move(flag), nullp.release(), move(p)), true };
  + return { expr::mkIf(move(flag), nullp.release(), move(p)), expr(np) };
  }
  expr Malloc::getTypeConstraints(const Function &f) const {
  