Introduced in commit 61c8fe6 from last month that moved the callback
into the job queue:
1. It leaked `fre->held_val` when no job was enqueued
2. It fumbled the reference count when enqueuing; JS_EnqueueJob already
takes care of incrementing and decrementing it
Reverts commit 0a70623 from earlier today because that didn't turn out
to be a complete fix.
Fixes: https://github.com/quickjs-ng/quickjs/issues/648
