Fix worker port use-after-free

[nickva]

In #462 during exit we call js_std_free_handlers() then JS_FreeRuntime().

js_std_free_handlers() frees the JSThreadState memory. However, later in JS_FreeRuntime() we run GC, which runs js_worker_finalizer and that accesses the port list from the already freed JSThreadState, so it results in a use-after-free error.

To fix it try to run GC in js_std_free_handlers() just before before cleaning and freeing JSThreadState.

Fix #462

[nickva]

Not sure if this entirely right, maybe it's better if the port list has a mutex and a reference count like message pipes...

It does not trigger the UAF from #463 any longer at least:

% make CONFIG_ASAN=n qjs && ./qjs --std ./uaf.js
make: `qjs' is up to date.
qjs(74987,0x7ff85a0d5400) malloc: nano zone abandoned due to inability to reserve vm space.
ReferenceError: 'gc' is not defined
    at set (./uaf.js:9:7)
    at <anonymous> (./uaf.js:14:4)
    at <anonymous> (./uaf.js:15:3)
ReferenceError: could not load module filename 'non-exist.js'

[bellard]

Already fixed.