Commit 0c03b505 was meant to clear all the fds indexed by j in
[0, n_fds), which socket_disconnect() can't be allowed to close
(because on failure the caller remains responsible for closing them);
but instead it closed the one we failed to add to the main loop
(fd i), repeatedly.

Similarly, it was meant to invalidate all the watches indexed by j
in [i, n_fds) (the one we failed to add to the main loop and the ones
we didn't try to add to the main loop yet), which socket_disconnect()
can't be allowed to see (because it would fail to remove them from
the main loop and hit an assertion failure); but instead it invalidated
fd i, repeatedly.

These happen to be the same thing if you only have one fd, resulting
in the test-case passing on an IPv4-only system, but failing on a
system with both IPv4 and IPv6.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=89104


Signed-off-by: Simon McVittie <smcv@collabora.com>
Reviewed-by: Philip Withnall <withnall@endlessm.com>