bug 1279984 - allow destroying proxies without a wrapper r=davidb

We can sometimes call ProxyDestroyed() on a proxy that never had a wrapper set
up so we should just bail out of ProxyDestroyed() in that case because there is
nothing to do.

accessible/atk/AccessibleWrap.cpp

@@ -1156,6 +1156,10 @@ void
 a11y::ProxyDestroyed(ProxyAccessible* aProxy)
 {
   auto obj = reinterpret_cast<MaiAtkObject*>(aProxy->GetWrapper() & ~IS_PROXY);
+  if (!obj) {
+    return;
+  }
+
   obj->Shutdown();
   g_object_unref(obj);
   aProxy->SetWrapper(0);