net/can: fix possible IOB resource leak when can close

If there is data in readahead, it needs to be released; otherwise,
it will cause the leakage of IOB resources

Signed-off-by: zhanghongyu <zhanghongyu@xiaomi.com>

net/can/can_conn.c

@@ -169,6 +169,10 @@ void can_free(FAR struct can_conn_s *conn)
   nxsem_destroy(&conn->sndsem);
 #endif
 
+  /* Free the readahead queue */
+
+  iob_free_queue(&conn->readahead);
+
   /* Free the connection. */
 
   NET_BUFPOOL_FREE(g_can_connections, conn);