[artwork] Use cache stash to avoid making repeated requests

E.g. for an album we would for each track request artwork from Spotify, even
though the requests are similar and the first one didn't give a result.

src/artwork.c

@@ -923,8 +923,11 @@ artwork_get_byurl(struct evbuffer *artwork, const char *url, int max_w, int max_
   CHECK_NULL(L_ART, raw = evbuffer_new());
 
   ret = cache_artwork_read(raw, url, &format);
-  if (ret == 0 && format > 0)
+  if (ret == 0)
     {
+      if (format <= 0)
+	goto error;
+
       ret = artwork_evbuf_rescale(artwork, raw, max_w, max_h);
       if (ret < 0)
 	goto error;