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>