Fix error when leaving Viewer

The carousel would go to a photo that doesn't exists

src/organisms/Carousel.svelte

@@ -121,7 +121,11 @@
 
         // Viewer: Navigate to photo on init and URL change
         if (viewer) {
-            page.subscribe(page => goToPhoto(photos.findIndex(photo => photo.slug === page.params.photo)))
+            page.subscribe(page => {
+                if (page.path.includes('/viewer/')) {
+                    goToPhoto(photos.findIndex(photo => photo.slug === page.params.photo))
+                }
+            })
         }
     })
 </script>