isl_tab_pip.c: sol_add: skip empty contexts

isl_tab_pip.c: sol_add: skip empty contexts

If the context becomes non-empty inside find_solutions, we stop
looking for solutions since a89841a (isl_tab_pip.c: find_solutions: break
when context becomes empty, Sun Jan 31 18:29:12 2010 +0100), but
we still call sol_add.  We need to return from sol_add early in
such cases because there is nothing to do and, more importantly,
when a basic set is marked empty, its existentials are removed,
resulting in a mismatch in the number of dimensions.  In particular,
the basic set returned by sol_domain may not satisfy all assumptions
made by the remainder of sol_add.

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