Fix GC leak in js_proxy_get()

[xeioex]

Simplified test case to reproduce:

function a() {
    return {};
}
var d = new Date();
var o1 = new Proxy(d, {get: 0});
var o2 = new Proxy(d, o1);
var o = new Proxy(o2, {get: a})

o.a

[saghul]

@chqrlie can we get this landed?