fix: use-after-free in free_msg_buffers_chunk_internal

C->free_block_queue was accessed after free(C). The memset(C,0,...)
before free() made this "work" (queue pointer was already NULL), but
it's still undefined behavior. Save the queue pointer before freeing
the chunk.

Found by -Werror CI job (GCC 13 -Wuse-after-free).

Author: dvershinin

net/net-msg-buffers.c

@@ -290,6 +290,8 @@ void free_msg_buffers_chunk_internal (struct msg_buffers_chunk *C, struct msg_bu
   __sync_fetch_and_add (&allocated_buffer_chunks, -1);
   MODULE_STAT->allocated_buffer_bytes -= MSG_BUFFERS_CHUNK_SIZE;
 
+  struct mp_queue *bq = C->free_block_queue;
+
   memset (C, 0, sizeof (struct msg_buffers_chunk));
   free (C);
 
@@ -302,9 +304,8 @@ void free_msg_buffers_chunk_internal (struct msg_buffers_chunk *C, struct msg_bu
   if (ChunkSave[si] == C) {
     ChunkSave[si] = NULL;
   }
-  
-  free_mp_queue (C->free_block_queue);
-  C->free_block_queue = NULL;
+
+  free_mp_queue (bq);
 }