qobject: Protect against use-after-free in qobject_decref()

Adding an assertion to qobject_decref() will ensure that a
programming error causing use-after-free will result in
immediate failure (provided no other thread has started
using the memory) instead of silently attempting to wrap
refcnt around and leaving the problem to potentially bite
later at a harder point to diagnose.

Backports commit cc9f60d4a2a4bf2578a9309a18f1c4602c9f5ce7 from qemu

qemu/include/qapi/qmp/qobject.h

@@ -94,6 +94,7 @@ static inline void qobject_incref(QObject *obj)
  */
 static inline void qobject_decref(QObject *obj)
 {
+    assert(!obj || obj->refcnt);
     if (obj && --obj->refcnt == 0) {
         assert(obj->type != NULL);
         assert(obj->type->destroy != NULL);