imsm: FIX: crash during getting map

imsm: FIX: crash during getting map

When get_imsm_map() is called with second_map parameter == '-1'
and array is not in migration state NULL pointer is returned.
This is wrong. '-1' means return map as migration record points.

'-1' can be passed to get_imsm_map() from imsm_num_data_members().
imsm_num_data_members() is called to get current map members based
on migr_state information

Signed-off-by: Adam Kwolek <adam.kwolek@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>