isl_tab_pip.c: no_sol_in_strict: fix error handling

isl_tab_pip.c: no_sol_in_strict: fix error handling

Ever since the context handling was extracted out in
isl-0.01-78-gfc0a2defe3 (isl_tab_pip.c: extract out context handling,
Thu Oct 8 13:49:57 2009 +0200), the add_ineq method
has never caused the isl_context pointer to become NULL.
The proper way of checking for errors is to call is_ok(),
which was introduced in the same commit.

Furthermore, ever since isl-0.01-79-gcfb1ea42e2 (isl_tab_pip:
add generalized basis reduction based context handling,
Thu Oct 8 14:10:30 2009 +0200), the save() method may perform
an allocation, which must be freed by calling restore()
(or discard()).

Signed-off-by: Sven Verdoolaege <sven.verdoolaege@gmail.com>