machine: GC machine when no leader PID is set

machine: GC machine when no leader PID is set

After d8854ff1aca4434db0d7d6dcaf9fcf2f38105fb4, 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
d8854ff1aca4434db0d7d6dcaf9fcf2f38105fb4 (v255) and
1762c2c045d3a78d3cad54c6b1e5ee9624b32b00 (v257).

Fixes #35602.