Don't panic if the passed url is empty

The proper way would be to return a 4xx error, but for now this
suffices.

go/favicon/favicon.go

@@ -66,7 +66,7 @@ func main() {
func HandleProxy(w http.ResponseWriter, r *http.Request) {
	w.Header().Set("Cache-Control", "max-age=2419200")
	url := r.URL.Query()["url"][0]
	url := r.URL.Query().Get("url")
	favicon, err := GetFaviconCached(url)
	if err != nil {
		fmt.Printf("Error: '%s': %s\n", url, err)
@@ -138,7 +138,7 @@ func GetImageCached(u string) ([]byte, string, error) {
}
func HandleGetFavicon(w http.ResponseWriter, r *http.Request) {
	url := r.URL.Query()["url"][0]
	url := r.URL.Query().Get("url")
	favicon, err := GetFaviconCached(url)
	if err != nil {
		fmt.Printf("Error: '%s': %s\n", url, err)