machine: GC machine when no leader PID is set

After d8854ff, if a serialized
leader PID of a machine is already dead when deserialized, then the
leader PID and PIDFD are unset for the machine, and subsequent
machine_start() -> machine_watch_pidfd() will trigger an assertion.
This makes a Machine object without leader PID GCed.

Fixes a bug introduced by the combination of
d8854ff (v255) and
1762c2c (v257).

Fixes #35602.

(cherry picked from commit 51a0a3b)
(cherry picked from commit 962bdd1)
(cherry picked from commit 0df46d1)

Author: yuwata

src/machine/machine.c

@@ -540,6 +540,9 @@ bool machine_may_gc(Machine *m, bool drop_not_started) {
         if (m->class == MACHINE_HOST)
                 return false;
 
+        if (!pidref_is_set(&m->leader))
+                return true;
+
         if (drop_not_started && !m->started)
                 return true;