Remove accidental superfluous braces; add test

The change in 36d6d91 turned out to not
fix the issue due to the extra braces. This fixes it and adds a test to
be sure.

lib/mdns_lite/responder.ex

@@ -90,7 +90,7 @@ defmodule MdnsLite.Responder do
   def stop_server(ifname, address) do
     GenServer.stop(via_name({ifname, address}))
   catch
-    {:exit, {:noproc, _}} ->
+    :exit, {:noproc, _} ->
       # Ignore if the server already stopped. It already exited due to the
       # network going down.
       :ok

test/mdns_lite/responder_test.exs

@@ -1,3 +1,9 @@
 defmodule MdnsLite.ResponderTest do
   use ExUnit.Case, async: false
+
+  alias MdnsLite.Responder
+
+  test "stopping a nonexistent responder" do
+    Responder.stop_server("no_exist", {1, 2, 3, 4})
+  end
 end