Prevent crash when trying to load a favicon from a non-existent URL

lib/frenzy/task/fetch_favicon.ex

@@ -98,9 +98,7 @@ defmodule Frenzy.Task.FetchFavicon do
     end
   end
 
-  defp fetch_favicon_data(nil), do: {:error, "No href for link"}
-
-  defp fetch_favicon_data(url) do
+  defp fetch_favicon_data(url) when is_binary(url) do
     case HTTP.get(url) do
       {:ok, %HTTPoison.Response{body: body}} ->
         {:ok, "data:image/png;base64,#{Base.encode64(body)}"}
@@ -110,6 +108,8 @@ defmodule Frenzy.Task.FetchFavicon do
     end
   end
 
+  defp fetch_favicon_data(_), do: {:error, "No or invalid href for link"}
+
   # from https://github.com/elixir-plug/plug/blob/v1.8.3/lib/plug/request_id.ex#L60
   defp generate_task_id() do
     binary = <<