QuickJS: fix fs callback return value leak

QuickJS: fix fs callback return value leak

Previously, the value returned by a callback passed to the callback
flavor of fs functions was never freed.  The value stayed referenced
until the runtime was freed, triggering the
"list_empty(&rt->gc_obj_list)" assertion in JS_FreeRuntime() in debug
builds and accumulating in the runtime with js_context_reuse.

The issue was introduced in 1f8f9992 (0.8.9).