message: fix memset to make poison ptrs always UB
  This is necessary as addresses must be sufficiently aligned, hence not poison
diff: @@ -3208,8 +3208,9 @@ StateValue Memset::toSMT(State &s) const {
  } else {
  auto &sv_ptr = s[*ptr];
  auto &sv_ptr2 = s[*ptr];
  - s.addUB((vbytes != 0).implies(
  - sv_ptr.non_poison && (sv_ptr.value == sv_ptr2.value)));
  + // can't be poison even if bytes=0 as address must be aligned regardless
  + s.addUB(sv_ptr.non_poison);
  + s.addUB((vbytes != 0).implies(sv_ptr.value == sv_ptr2.value));
  vptr = sv_ptr.value;
  }
  check_can_store(s, vptr);
  