Incorrect result after NMS in SSD object detection

[matt-ny]

This change https://github.com/intel/caffe/blame/7010334f159da247db3fe3a9d96a3116ca06b09a/src/caffe/util/bbox_util.cpp#L2266 using at instead of push_back is selectively over-writing some results, and leaving some low-scoring results within the top-k entries if there are not many strong detections in the image.

A fix is proposed in #261