core/cgroup: CGContext must be available if Unit has CGRuntime

Follow-up for e03e5056db

src/core/cgroup.c

@@ -3050,9 +3050,7 @@ int unit_check_oom(Unit *u) {
         if (!crt || !crt->cgroup_path)
                 return 0;
 
-        CGroupContext *ctx = unit_get_cgroup_context(u);
-        if (!ctx)
-                return 0;
+        CGroupContext *ctx = ASSERT_PTR(unit_get_cgroup_context(u));
 
         /* If memory.oom.group=1, then look up the oom_group_kill field, which reports how many times the
          * kernel killed every process recursively in this cgroup and its descendants, similar to

src/core/unit.c

@@ -5141,12 +5141,11 @@ int unit_setup_exec_runtime(Unit *u) {
         return r;
 }
 
-CGroupRuntime *unit_setup_cgroup_runtime(Unit *u) {
-        size_t offset;
-
+CGroupRuntime* unit_setup_cgroup_runtime(Unit *u) {
         assert(u);
+        assert(UNIT_HAS_CGROUP_CONTEXT(u));
 
-        offset = UNIT_VTABLE(u)->cgroup_runtime_offset;
+        size_t offset = UNIT_VTABLE(u)->cgroup_runtime_offset;
         assert(offset > 0);
 
         CGroupRuntime **rt = (CGroupRuntime**) ((uint8_t*) u + offset);