Fixed tracking for OpenCV 3.x

AlexeyAB

2018-01-11 89d0c46bb32a98416fb55a9c83c291dd0831c6cc

 src/network.c

@@ -218,6 +218,7 @@
            state.delta = prev.delta;
        }
        layer l = net.layers[i];
        if (l.stopbackward) break;
        l.backward(l, state);
    }
}