server crash on heartbeat due to missing message type

when client sends a heartbeat message the server logs:

server.c:319:client_thread(): ERROR: unknown message type: 7 and freezes my mining!

fix:

add following case to the switch in client_thread():

    case COIN_MSG__ENVELOPE__BODY_HEARTBEAT:
        pb_handle_heartbeat(fd, envelope->heartbeat, this_user);
        break;