Once the task is completed (and `g_task_return_*()` has been called),
the task is no longer needed. It would make more sense to unref it in
`complete_connection_with_error()`, where `g_task_return_*()` is called,
but that complicates other call sites significantly, so I didn’t do it.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Fixes: #3184