I know this is an old issue, but in my case, I found out that if the image had an impair height, that line appeared.
So I have done the following modifications to make it work for me.
public void reshape(ViewPort vp, int w, int h) {
outBuf = BufferUtils.createByteBuffer(w*h*4);
if (h % 2 == 1) {
h = h - 1;
}
screenShot = new BufferedImage(w, h, BufferedImage.TYPE_4BYTE_ABGR);
}