zsmalloc: fix a null pointer dereference in destroy_handle_cache()

If zs_create_pool()->create_handle_cache()->kmem_cache_create() or
pool->name allocation fails, zs_create_pool()->destroy_handle_cache()
will dereference the NULL pool->handle_cachep.

Modify destroy_handle_cache() to avoid this.
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/zsmalloc.c

@@ -289,6 +289,7 @@ static int create_handle_cache(struct zs_pool *pool)
 
 static void destroy_handle_cache(struct zs_pool *pool)
 {
+	if (pool->handle_cachep)
 		kmem_cache_destroy(pool->handle_cachep);
 }