snapshot: fix warning for discarding a nodiscard (#728)

src/entt/entity/snapshot.hpp

@@ -55,7 +55,7 @@ class basic_snapshot {
 
         while(begin != last) {
             const auto entt = *(begin++);
-            ((reg->template all_of<Component>(entt) ? ++size[Index] : size[Index]), ...);
+            ((reg->template all_of<Component>(entt) ? ++size[Index] : 0u), ...);
         }
 
         (get<Component>(archive, size[Index], first, last), ...);