home: fix heap-use-after-free

`bus_home_emit_remove()` may be called from manager_free() -> home_free().
In that case, manager->bus is already unref()ed.

Fixes #21767.

src/home/homed-home-bus.c

@@ -940,6 +940,12 @@ int bus_home_emit_remove(Home *h) {
         if (!h->announced)
                 return 0;
 
+        if (!h->manager)
+                return 0;
+
+        if (!h->manager->bus)
+                return 0;
+
         r = bus_home_path(h, &path);
         if (r < 0)
                 return r;