isl_multi_union_pw_aff_from_multi_pw_aff: avoid risk of spurious silent errors
isl_multi_union_pw_aff_from_multi_pw_aff: avoid risk of spurious silent errors

The function isl_multi_union_pw_aff_from_multi_pw_aff initializes
the isl_multi_pw_aff it is constructing using isl_multi_union_pw_aff_alloc,
meaning the entries are set to NULL.
Subsequently calling isl_multi_union_pw_aff_set_union_pw_aff
risks having the NULL entries being treated as (silent) errors when
the parameters do not align.
Call isl_multi_union_pw_aff_restore_check_space instead,
which explicitly allows entries to be NULL.
It never does any parameter alignment, but it still checks
that they align already, vocally erroring out when they do not.

This is similar to the change in isl-0.19-11-gd9063ab6a4
(isl_multi_*_from_*_list: use isl_multi_*_restore,
Thu May 17 14:59:28 2018 +0200), fixed up further in
isl-0.19-13-g3c70ee71c5 (isl_multi_*_from_*_list:
reintroduce space compatibility check, Wed May 30 10:41:48 2018 +0200).

Signed-off-by: Sven Verdoolaege <235c10dd23b819f81cdc9756a251746bc184cab6@gmail.com>

Meinersbur/isl,Meinersbur/isl,Meinersbur/isl,Meinersbur/isl