block: avoid to READ fields of null bio

rq->bio can be NULL sometimes, such as flush request, so don't
read bio->bi_seg_front_size until this 'bio' is checked as valid.

Cc: Bart Van Assche <bvanassche@acm.org>
Reported-by: Bart Van Assche <bvanassche@acm.org>
Fixes: dcebd755 ("block: use bio_for_each_bvec() to compute multi-page bvec count")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-merge.c

@@ -363,13 +363,15 @@ static unsigned int __blk_recalc_rq_segments(struct request_queue *q,
 	struct bio_vec bv, bvprv = { NULL };
 	int prev = 0;
 	unsigned int seg_size, nr_phys_segs;
-	unsigned front_seg_size = bio->bi_seg_front_size;
+	unsigned front_seg_size;
 	struct bio *fbio, *bbio;
 	struct bvec_iter iter;
 
 	if (!bio)
 		return 0;
 
+	front_seg_size = bio->bi_seg_front_size;
+
 	switch (bio_op(bio)) {
 	case REQ_OP_DISCARD:
 	case REQ_OP_SECURE_ERASE: