logind: drop the seat from the GC queue after draining its devices

logind: drop the seat from the GC queue after draining its devices

manager_gc() pops a seat off m->seat_gc_queue and clears
seat->in_gc_queue before calling seat_free(). seat_free() then drains
the seat's devices, and device_detach() calls seat_add_to_gc_queue() for
a seat that just lost its last master device, re-prepending the dying
seat onto m->seat_gc_queue. seat_free() removed itself from the queue up
front, before that drain, so the re-queued seat is left on the queue as a
dangling pointer once mfree() runs.

Follow-up for 718d006a63f773c42106494e823250c48942cf08