Code bug in MozValue.cc

[enricogior]

This code bug was found while investigating another bug (thaliproject/Thali_CordovaPlugin#1592).

  JS::Heap<JS::Value> hval;
  hval = JS::ObjectOrNullValue(reserved_obj);

  JS::AddNamedValueRoot(ctx_, &hval, nullptr);
  JS_SetReservedSlot(object_, GC_SLOT_GC_CALL, hval);

https://github.com/thaliproject/jxcore/blob/master/src/jx/Proxy/Mozilla_340/MozJS/MozValue.cc#L1151-L1155

hval is allocated on the stack and then passed to AddNamedValueRoot that is expecting a JS::Value object allocated on the heap.

The bug is low priority since that code path is not currently used by the ThaliProject code or any JXcore test.