ocfs2: fix null pointer dereference in ocfs2_dir_foreach_blk_id()

Fix a NULL pointer deference while removing an empty directory, which
was introduced by commit 3704412b

 ("[readdir] convert ocfs2").

  BUG: unable to handle kernel NULL pointer dereference at (null)
  IP: [<(null)>]           (null)
  PGD 6da85067 PUD 6da89067 PMD 0
  Oops: 0010 [#1] SMP
  CPU: 0 PID: 6564 Comm: rmdir Tainted: G           O 3.11.0-rc1 #4
  RIP: 0010:[<0000000000000000>]  [<          (null)>]           (null)
  Call Trace:
    ocfs2_dir_foreach+0x49/0x50 [ocfs2]
    ocfs2_empty_dir+0x12c/0x3e0 [ocfs2]
    ocfs2_unlink+0x56e/0xc10 [ocfs2]
    vfs_rmdir+0xd5/0x140
    do_rmdir+0x1cb/0x1e0
    SyS_rmdir+0x16/0x20
    system_call_fastpath+0x16/0x1b
  Code:  Bad RIP value.
  RIP  [<          (null)>]           (null)
  RSP <ffff88006daddc10>
  CR2: 0000000000000000

[dan.carpenter@oracle.com: fix pointer math]
Signed-off-by: Jie Liu <jeff.liu@oracle.com>
Reported-by: David Weber <wb@munzinger.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Mark Fasheh <mfasheh@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>