bsg: fix potential error pointer dereference

Dan writes:

block/bsg.c:327 bsg_map_hdr() error: 'next_rq' dereferencing possible
ERR_PTR().

Fix this by setting next_rq to NULL, for the case where it can be
!= NULL but an error pointer.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

block/bsg.c

@@ -287,6 +287,7 @@ bsg_map_hdr(struct bsg_device *bd, struct sg_io_v4 *hdr, fmode_t has_write_perm,
 		next_rq = blk_get_request(q, READ, GFP_KERNEL);
 		if (IS_ERR(next_rq)) {
 			ret = PTR_ERR(next_rq);
+			next_rq = NULL;
 			goto out;
 		}
 		rq->next_rq = next_rq;