fix invalid video poster image loading from panicking (#31447)

This fixed #31438. When an image is loaded from cache, it will
load a placeholder if the url is not a valid image url. This
may be unexpected when using the image cache and specifying
UsePlaceholder::No but that has no effect on loading an image
not in the cache.

Signed-off-by: Sebastian C <sebsebmc@gmail.com>

components/script/dom/htmlvideoelement.rs

@@ -302,8 +302,8 @@ impl ImageCacheListener for HTMLVideoElement {
                 LoadBlocker::terminate(&mut *self.load_blocker.borrow_mut());
             },
             ImageResponse::MetadataLoaded(..) => {},
-            ImageResponse::PlaceholderLoaded(..) => unreachable!(),
-            ImageResponse::None => {
+            // The image cache may have loaded a placeholder for an invalid poster url
+            ImageResponse::PlaceholderLoaded(..) | ImageResponse::None => {
                 // A failed load should unblock the document load.
                 LoadBlocker::terminate(&mut *self.load_blocker.borrow_mut());
             },