Fix concurrency bug

[halfer53]

if(is_debugging_ipc()){
                KDEBUG(("IPC: SEND to %d from %d blocked\n",
                            dest, src->proc_nr));
            }
            // Otherwise, block current process and add it to
            // head of sending queue of the destination.
            src->state |= STATE_SENDING;
            src->next_sender = pDest->sender_q;
            pDest->sender_q = src;

This part could trigger undefined behavior if the sender is already receiving