Make sure the `GTask` is freed on a graceful disconnect
This fixes the memory leak in the case the connection has been successfully closed by the peer.
Check with
ninja && valgrind \
--leak-check=full \
--show-leak-kinds=definite \
--suppressions=../tools/glib.supp \
gio/tests/send-data --async --graceful-disconnect gnome.org:80
Press Ctrl+D after the connection is established.