System information (version)

• OpenCV => 3.4.5
• Operating System / Platform => Mac OS X
• Compiler => cv2.error: OpenCV(3.4.5)

Detailed description

1. I wrote the model of PNet into freeze pb, the load the mode by readNetFromTensorflow. Then it occur error . I tried for RNet and ONet, then load correctly.
2. Does the dnn do not support load multi-input nodes ? I modify the input of the pb as only one input, error occor as the same.

Steps to reproduce

```.python
    pb_file = "PNet_pnet.pb"

    net = cv.dnn.readNetFromTensorflow(pb_file)
    print(net)
```

cv2.error: OpenCV(3.4.5) /Users/travis/build/skvark/opencv-python/opencv/modules/dnn/src/tensorflow/tf_importer.cpp:522: error: (-2:Unspecified error) More than one input is Const op in function 'getConstBlob'

PNet_pnet.pb

Appreciate for your help.