Component model async + epoch interruption is broken

[dicej]

I tried experimenting with wasi:http@0.3.0-draft with epoch interruption enabled the other day, and it segfaulted intermittently. I tracked that down to a null pointer dereference in concurrent.rs's AsyncCx implementation and added an assertion to turn the segfault into a panic, but haven't gone further than that. Hopefully it should be easy to fix.

[dicej]

This should be fixed as of #13