LibSQL: Add a note to Serializer about m_heap

Author: https://github.com/gmta
Commit: 71c98546b8
Pull-request: https://github.com/SerenityOS/serenity/pull/18476
Reviewed-by: https://github.com/trflynn89 ✅

Userland/Libraries/LibSQL/Serializer.h

@@ -160,6 +160,7 @@ private:
 
     ByteBuffer m_buffer {};
     size_t m_current_offset { 0 };
+    // FIXME: make this a NonnullRefPtr<Heap> so we can get rid of the null checks
     RefPtr<Heap> m_heap { nullptr };
 };