ubi: Add a check for ubi_num

ubi: Add a check for ubi_num

Added a check for ubi_num for negative numbers
If the variable ubi_num takes negative values then we get:

qemu-system-arm ... -append "ubi.mtd=0,0,0,-22222345" ...
[    0.745065]  ubi_attach_mtd_dev from ubi_init+0x178/0x218
[    0.745230]  ubi_init from do_one_initcall+0x70/0x1ac
[    0.745344]  do_one_initcall from kernel_init_freeable+0x198/0x224
[    0.745474]  kernel_init_freeable from kernel_init+0x18/0x134
[    0.745600]  kernel_init from ret_from_fork+0x14/0x28
[    0.745727] Exception stack(0x90015fb0 to 0x90015ff8)

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 83ff59a06663 ("UBI: support ubi_num on mtd.ubi command line")
Cc: stable@vger.kernel.org
Signed-off-by: Denis Arefev <arefev@swemel.ru>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>