message: fix underflow in Pointer::isConstGlobal()
  can be triggered when num_nonlocals_src is a power of two
diff: @@ -170,7 +170,7 @@ expr Pointer::isConstGlobal() const {
  auto generic = bid.uge(has_null_block) &&
  expr(num_consts_src > 0) &&
  bid.ule(num_consts_src + has_null_block - 1);
  - auto tgt = bid.uge(num_nonlocals_src) && bid.ule(num_nonlocals - 1);
  + auto tgt = num_nonlocals_src == 0 ? expr(true) : bid.ugt(num_nonlocals_src-1);
  return !isLocal() && (generic || tgt);
  }
  