core/manager: do not pop gc_unit_queue before unit_gc_sweep()

Follow-up for 52e3671

unit_gc_sweep() might try to add the unit to gc queue again.
While that becomes no-op as Unit.in_gc_queue is not cleared
yet, it induces minor inconsistency of states.

(cherry picked from commit 741a184)

Author: YHNdnzj

src/core/manager.c

@@ -1474,7 +1474,6 @@ static void unit_gc_sweep(Unit *u, unsigned gc_marker) {
 
 static unsigned manager_dispatch_gc_unit_queue(Manager *m) {
         unsigned n = 0, gc_marker;
-        Unit *u;
 
         assert(m);
 
@@ -1486,11 +1485,13 @@ static unsigned manager_dispatch_gc_unit_queue(Manager *m) {
 
         gc_marker = m->gc_marker;
 
-        while ((u = LIST_POP(gc_queue, m->gc_unit_queue))) {
+        Unit *u;
+        while ((u = m->gc_unit_queue)) {
                 assert(u->in_gc_queue);
 
                 unit_gc_sweep(u, gc_marker);
 
+                LIST_REMOVE(gc_queue, m->gc_unit_queue, u);
                 u->in_gc_queue = false;
 
                 n++;