sixlowpan: decrement processing semaphore when discarding

sys/net/network_layer/sixlowpan/mac.c

@@ -150,6 +150,7 @@ static void *recv_ieee802154_frame(void *arg)
             }
             else {
                 DEBUG("Unknown IEEE 802.15.4 destination address mode.\n");
+                p->processing--;
                 continue;
             }