Fix #354: Signed Integer Overflow gd_io.c

GD2 stores the number of horizontal and vertical chunks as words (i.e. 2
byte unsigned). These values are multiplied and assigned to an int when
reading the image, what can cause integer overflows. We have to avoid
that, and also make sure that either chunk count is actually greater
than zero. If illegal chunk counts are detected, we bail out from
reading the image.
BGD_DECLARE(void *) gdImageGd2Ptr (gdImagePtr im, int cs, int fmt, int *size)
{
	_noLibzError();
	return NULL;
}

BGD_DECLARE(void *) gdImageGd2Ptr (gdImagePtr im, int cs, int fmt, int *size)
{
	_noLibzError();
	return NULL;
}
