Merge pull request #711 from debug-ito/tsch-handle-prepare-error

tsch: handle failure in prepare()

os/net/mac/tsch/tsch-slot-operation.c

@@ -643,6 +643,8 @@ PT_THREAD(tsch_tx_slot(struct pt *pt, struct rtimer *t))
             mac_tx_status = MAC_TX_ERR;
           }
         }
+      } else {
+        mac_tx_status = MAC_TX_ERR;
       }
     }