vm: minimum and maximum zone IDs are now defined in arch/../vm.h

vm/cache.c

@@ -82,6 +82,10 @@ void vm_cache_destroy(struct vm_cache *cache)
 static struct vm_slab *alloc_slab(struct vm_cache *cache, enum vm_flags flags)
 {
 	struct vm_page *slab_page = vm_page_alloc(cache->c_page_order, flags);
+	if (!slab_page) {
+		return NULL;
+	}
+
 	struct vm_slab *slab_hdr = NULL;
 	void *slab_data = vm_page_get_vaddr(slab_page);