imsm: assert if there is migration but prev_map doesn't exist

imsm: assert if there is migration but prev_map doesn't exist

Verify that prev_map in not null during volume migration.
Practically this case is not possible, device prev_map is being
added if it is in the middle of migration.
Add verification to silence static code analyze errors.

Change error handling for function is_gen_migration() (as well as
values compared with return value from this function) to use boolean
types provided by stdbool.h.

Signed-off-by: Pawel Piatkowski <pawel.piatkowski@intel.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>