iterate through less detections if returned less than max amount

[rrtaylor]

In tflite models other than the one provided in the example (e.g. ssd_mobilenet_v1_coco), there may be less detections returned than NUM_DETECTIONS. When less detections are provided, garbage float values are passed back, causing the example to crash or return false predictions. This is prevented if you use the returned numDetections value to help control the number of detections to use.

fixes https://github.com/tensorflow/tensorflow/issues/22278 and https://github.com/tensorflow/tensorflow/issues/22106.