imsm: fix thunderdome segfault

imsm: fix thunderdome segfault

disk_list_get() can return NULL if:
1/ A formerly missing disk is re-added
2/ The original array has not been rebuilt, so the family number of the
   missing disk still matches
3/ The metadata record of the in-sync disks are read before the missing
   disk

This will result in the missing disk not adding its own serial number to
the disk_list, only its truncated value will be present.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>