get rid of notices when $node->images[IMAGE_ORIGINAL] is not set

simple patch, just getting rid of a notice, was trying to setup a gallery to work on #61704: setting up Custom Pagers to allow browsing next/previous image in gallery.

+  // Its possible there was an error creating the image, so don't assume 
+  // IMAGE_ORIGINAL is set.
+  $original_path = isset($node->images[IMAGE_ORIGINAL]) ? $node->images[IMAGE_ORIGINAL] : NULL;