isl_basic_map_foreach_lexopt: properly merge partial empty solutions

isl_basic_map_foreach_lexopt: properly merge partial empty solutions

b9f95db (isl_basic_map_foreach_lexopt: properly merge partial solutions,
Wed Mar 20 09:55:50 2013 +0100) fixed the merging of partial non-empty
solutions, but did not take into account that partial solutions may
be empty.  If the partial solutions are empty, then M is NULL and we
cannot dereference it.

Note that the new test case only triggers on top of d7e5adb
(isl_tab_basic_map_partial_lexopt: better exploit partial solution cache,
Tue Mar 12 17:25:04 2013 +0100) in the master branch.

Reported-by: Tomofumi Yuki <tomofumi.yuki@gmail.com>
Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>