quickfix #9903 by moving assert into async match branches

[AaronO]

This is a quickfix for #9903. It moves the assert into the async branches.

However the shared queue should be dropped very soon and will also fix this issue.

[inteon]

@AaronO I think this might still cause failure, I propose to just remove the check.
This will be the case if (adaptation on original bug-report):

• Two async Ops resolve at the exact same time (between two polls of the JsRuntime)
• An async op is dispatched in the async callback for the first async op

[fasterthanlime]

@inteon Yup, I can confirm that the quickfix does not address the variant you outlined in #9904 (comment) (I have a local branch of a proprietary codebase I was able to tweak to test that case).

Removing the shared queue sounds good, happy to wait for that — we can live with a fork+cargo-patch for now.