Avoid potential division by zero when performing the check against
multiplication overflow. (Note that if the width is zero then there
can be no overflow anyway, so it is then safe to bypass the check.)
Signed-off-by: Michael Brown <mcb30@ipxe.org>
Enable IMAGE_PNG (but not IMAGE_PNM) by default, and drag in the
relevant objects only when image_pixbuf() is present in the binary.
Signed-off-by: Michael Brown <mcb30@ipxe.org>
