blk-mq: Add a NULL check in blk_mq_free_map_and_requests()

I recently found some code which called blk_mq_free_map_and_requests()
with a NULL set->tags pointer.  I fixed the caller, but it seems like a
good idea to add a NULL check here as well.  Now we can call:

	blk_mq_free_tag_set(set);
	blk_mq_free_tag_set(set);

twice in a row and it's harmless.
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq.c

@@ -2341,7 +2341,7 @@ static bool __blk_mq_alloc_rq_map(struct blk_mq_tag_set *set, int hctx_idx)
 static void blk_mq_free_map_and_requests(struct blk_mq_tag_set *set,
 					 unsigned int hctx_idx)
 {
-	if (set->tags[hctx_idx]) {
+	if (set->tags && set->tags[hctx_idx]) {
 		blk_mq_free_rqs(set, set->tags[hctx_idx], hctx_idx);
 		blk_mq_free_rq_map(set->tags[hctx_idx]);
 		set->tags[hctx_idx] = NULL;