memcontrol: fix potential null deref

There was a potential null deref introduced in c62b1a3b ("memcg: use
generic percpu instead of private implementation").
Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -3691,7 +3691,9 @@ static struct mem_cgroup *mem_cgroup_alloc(void)
 	else
 		mem = vmalloc(size);
 
-	if (mem)
+	if (!mem)
+		return NULL;
+
 	memset(mem, 0, size);
 	mem->stat = alloc_percpu(struct mem_cgroup_stat_cpu);
 	if (!mem->stat) {