message: fix Pointer::isBlockAlive() for null blocks
diff: @@ -715,7 +715,7 @@ expr Pointer::isBlockAlive() const {
  return true;
  // NULL block is dead
  - if (num_nonlocals && getBid().isZero())
  + if (has_null_block && getBid().isZero())
  return false;
  auto bid = getShortBid();
  