vk_reosurce_manager: Add assert for releasing fences

Notify the programmer when a request to release a fence is invalid
because the fence is already free.

src/video_core/renderer_vulkan/vk_resource_manager.cpp

@@ -87,6 +87,7 @@ void VKFence::Wait() {
 }
 
 void VKFence::Release() {
+    ASSERT(is_owned);
     is_owned = false;
 }