Investigating hexops/zorex#4, I found that `--test-evented-io` is currently broken in
the latest Zig nightly. See #9779 for a small reproduction.

The issue is that allocation errors here are not correctly handled, as this function
returns `void` and all other error cases `@panic`, the allocation failure should also
use `@panic`.

Fixes #9779
Helps hexops/zorex#4

Signed-off-by: Stephen Gutekanst <stephen@hexops.com>