fetcher: Close request body on error paths

Otherwise, we're potentially holding up subsequent requests.

I was hitting this when testing the metalink code, where we want to
continue doing more fetches after hitting a 404.

https://bugzilla.gnome.org/show_bug.cgi?id=729585

src/libostree/ostree-fetcher.c

@@ -505,6 +505,8 @@ on_request_sent (GObject        *object,
  out:
   if (local_error)
     {
+      if (pending->request_body)
+        (void) g_input_stream_close (pending->request_body, NULL, NULL);
       g_simple_async_result_take_error (pending->result, local_error);
       g_simple_async_result_complete (pending->result);
       g_object_unref (pending->result);