mtd: core: check partition before dereference

syzbot is reporting NULL pointer dereference at mtd_check_of_node() [1],
for mtdram test device (CONFIG_MTD_MTDRAM) is not partition.

Link: https://syzkaller.appspot.com/bug?extid=fe013f55a2814a9e8cfd [1]
Reported-by: syzbot <syzbot+fe013f55a2814a9e8cfd@syzkaller.appspotmail.com>
Reported-by: kernel test robot <oliver.sang@intel.com>
Fixes: ad9b10d1

 ("mtd: core: introduce of support for dynamic partitions")
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
CC: stable@vger.kernel.org
Signed-off-by: Richard Weinberger <richard@nod.at>