When calling g_socket_listener_accept_socket_async() on a
GSocketListener with multiple sockets, the accept_ready() callback is
called for the first incoming connection on each socket. It will return
success/failure for the entire accept_socket_async() GTask, and then
free the GSources for listening for incoming connections on the other
sockets in the GSocketListener. The GSources are freed when the GTask is
finalised.

However, if incoming connections arrive for multiple sockets within the
same GMainContext iteration, accept_ready() will be called multiple
times, and will call g_task_return_*() multiple times, before the GTask
is finalised. Calling g_task_return_*() multiple times is not allowed.

Propagate the first success/failure, as before, but then ignore all
subsequent incoming connections until the GTask is finalised.

Signed-off-by: Philip Withnall <withnall@endlessm.com>