fix the bug of postprocess

[iloveai8086]

Sometimes there is no object in my pointcloud,The following codes in postprocess_kernels.cu:
if (dev_cls[1] >= score_thresh)
{
int box_offset = loc_index * num_anchors * num_box_values + ith_anchor * num_box_values;
int dir_cls_offset = loc_index * num_anchors * 2 + ith_anchor * 2;
float *anchor_ptr = anchors + ith_anchor * 4;
float z_offset = anchor_ptr[2] / 2 + anchor_bottom_heights[ith_anchor / 2];
float anchor[7] = {x_offset, y_offset, z_offset, anchor_ptr[0], anchor_ptr[1], anchor_ptr[2], anchor_ptr[3]};
float *box_encodings = box_input + box_offset;
..............
When the scores of all targets are less than the threshold,the code will not run in this branch,the value of bndbox_output will not be updated, Instead, the result of the previous frame is used.
So,Before entering the kernel, I use "bndbox_output[0]=0;" to prevent unnecessary output in pointpillar.cpp.