async_wrap: clear destroy_ids vector

After processing all the callbacks in the destroy_ids vector make sure
to clear() it otherwise the DestroyIdsCb() won't run again.

PR-URL: https://github.com/nodejs/node/pull/10400
Fixes: b49b496a

 "async_wrap: call destroy() callback in uv_idle_t"
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anna Henningsen <anna@addaleax.net>