Do not free cf_image_t data structure in _cfImageZoomDelete()

The library-internal _cfImageZoom...() API handles zooming of images
with a data structure of type cf_izoom_t. This data structure contains
a pointer to the original image, in cf_image_t format.

The _cfImageZoomNew() funtion gets a pointer to an existing cf_image_t
structure as parameter, the image to work on. It stores the pointer in
its cf_izoom_t structure. It never creates a cf_image_t image.

Because the _cfImageZoom...() API never creates a cf_image_t
structure, it should also never free it. Therefore it is wrong that
_cfImageZoomDelete() calls cfImageClose() which is the function to
free a cf_image_t structure after use.

This was leading to double freeing, as the functions which create a
cf_image_t structure always free it when done.

This caused OpenPrinting/cups-filters#507

cupsfilters/image-zoom.c

@@ -42,7 +42,6 @@ _cfImageZoomDelete(cf_izoom_t *z)	// I - Zoom record to free
   free(z->rows[0]);
   free(z->rows[1]);
   free(z->in);
-  cfImageClose(z->img);
   free(z);
 }