fix: panic on transform stream check

[guybedford]

This fixes a panic Fastly was seeing when passing a request body to a response, where this slot was not a boolean causing a panic.

[cfallin]

This seems locally fine as a more robust check, but do we know why the slot was not a boolean? Is there a path that doesn't initialize it, or is some other value being stored there erroneously?

[guybedford]

There's a lot of complex logic here and quite honestly I haven't traced it carefully, but I can verify this solves the full end to end Fastly issue in fastly/js-compute-runtime#954.