check_visual_pos(void)
{
    if (VIsual.lnum > curbuf->b_ml.ml_line_count)
    {
	VIsual.lnum = curbuf->b_ml.ml_line_count;
	VIsual.col = 0;
	VIsual.coladd = 0;
    }
    else
    {
	int len = (int)STRLEN(ml_get(VIsual.lnum));

	if (VIsual.col > len)
	{
	    VIsual.col = len;
	    VIsual.coladd = 0;
	}
    }
}
The program copies an input buffer to an output buffer without verifying that the size of the input buffer is less than the size of the output buffer, leading to a buffer overflow.