libnvdimm, namespace: potential NULL deref on allocation error

If the kcalloc() fails then "devs" can be NULL and we dereference it
checking "devs[i]".

Fixes: 1b40e09a ('libnvdimm: blk labels and namespace instantiation')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>