varlink: avoid using dangling ref in varlink_close_unref()

varlink: avoid using dangling ref in varlink_close_unref()

Fixes #18025, https://bugzilla.redhat.com/show_bug.cgi?id=1931034.

We drop the reference stored in Manager.managed_oom_varlink_request in two code paths:
vl_disconnect() which is installed as a disconnect callback, and in manager_varlink_done().
But we also make a disconnect from manager_varlink_done(). So we end up with the following
call stack:

(gdb) bt
 0  vl_disconnect (s=0x112c7b0, link=0xea0070, userdata=0xe9bcc0) at ../src/core/core-varlink.c:414
 1  0x00007f1366e9d5ac in varlink_detach_server (v=0xea0070) at ../src/shared/varlink.c:1210
 2  0x00007f1366e9d664 in varlink_close (v=0xea0070) at ../src/shared/varlink.c:1228
 3  0x00007f1366e9d6b5 in varlink_close_unref (v=0xea0070) at ../src/shared/varlink.c:1240
 4  0x0000000000524629 in manager_varlink_done (m=0xe9bcc0) at ../src/core/core-varlink.c:479
 5  0x000000000048ef7b in manager_free (m=0xe9bcc0) at ../src/core/manager.c:1357
 6  0x000000000042602c in main (argc=5, argv=0x7fff439c43d8) at ../src/core/main.c:2909

When we enter vl_disconnect(), m->managed_oom_varlink_request.n_ref==1.
When we exit from vl_discconect(), m->managed_oom_varlink_request==NULL. But
varlink_close_unref() has a copy of the pointer in *v. When we continue executing
varlink_close_unref(), this pointer is dangling, and the call to varlink_unref()
is done with an invalid pointer.