CVE-2021-39216: Use after free passing `externref`s to Wasm in Wasmtime

There was a use-after-free bug when passing externrefs from the host to guest Wasm content.

To trigger the bug, you have to explicitly pass multiple externrefs from the host to a Wasm instance at the same time, either by

• passing multiple externrefs as arguments from host code to a Wasm function,
• or returning multiple externrefs to Wasm from a multi-value return function defined in the host.

If you do not have host code that matches one of these shapes, then you are not impacted.