Assemble: set devices to NULL when load_devices can't load device

Assemble: set devices to NULL when load_devices can't load device

Since load_devices frees "devices" when it can't find any
device, we should set it to NULL to avoid double free issue
which can be reproduced by below steps:

mdadm -CR /dev/md/vol -l0 -e 1.2 -n2 /dev/sd[b-c] --assume-clean
mdadm -Ss
mdadm -A /dev/md127 /dev/sd[b-c] --update metadata

Reported-by: Tkaczyk Mariusz <mariusz.tkaczyk@intel.com>
Tested-by: Tkaczyk Mariusz <mariusz.tkaczyk@intel.com>
Signed-off-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>