calloc: Use calloc() at most places
    
    This modifies most of the places we do some form of:
    
      X = malloc(Y * Z);
    
    to use calloc(Y, Z) instead.
    
    Among other issues, this fixes:
      - allocation of integer overflow in grub_png_decode_image_header()
        reported by Chris Coulson,
      - allocation of integer overflow in luks_recover_key()
        reported by Chris Coulson,
      - allocation of integer overflow in grub_lvm_detect()
        reported by Chris Coulson.
    
    Fixes: CVE-2020-14308
    
    Signed-off-by: Peter Jones <pjones@redhat.com>
    Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>