isl_convex_hull.c: convex_hull_pair: add missing return

The problem was introduced all the way back in 7f46696 (isl_convex_hull.c:
convex_hull_pair: skip direction change if result is bounded,
Mon Apr 19 09:23:55 2010 +0200).  Apparently, the code hardly ever
gets executed, which is not entirely unexpected as it is only executed
when the first two basic sets considered in an unbounded set are both
bounded.

Reported-by: antoine.morvan.pro@gmail.com
Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>
isl_input.c: basic_map_read: check type of result returned by obj_read

Ever since 67fc9fa (add isl_stream_read_obj, Fri Feb 19 13:11:21 2010 +0100),
we simply assume that this result is either a set or a map (or NULL),
which was true back then, but since then obj_read has been modified
to return several different kinds of objects so we need to check
if the returned object is indeed a set or a map.

Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>