WASM: Fix `Wasm_pvm.get_output`

Context

During a peer programming session with @emturner and @corneliuhoffman, we noticed an issue in the get_output function of the WASM PVM. As far as I can tell, it’s still not fixed in master, so here is a quick MR to deal with the issue.

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR