core/slice: make slice_freezer_action() return 0 if freezing state is unchanged

Fixes #23278.

src/core/slice.c

@@ -389,11 +389,7 @@ static int slice_freezer_action(Unit *s, FreezerAction action) {
                         return r;
         }
 
-        r = unit_cgroup_freezer_action(s, action);
-        if (r < 0)
-                return r;
-
-        return 1;
+        return unit_cgroup_freezer_action(s, action);
 }
 
 static int slice_freeze(Unit *s) {

src/core/unit.c

@@ -5849,6 +5849,8 @@ static int unit_freezer_action(Unit *u, FreezerAction action) {
         if (r <= 0)
                 return r;
 
+        assert(IN_SET(u->freezer_state, FREEZER_FREEZING, FREEZER_THAWING));
+
         return 1;
 }