isl_sample.c: tab_shift_cone: use isl_tab_extend_cons instead of isl_tab_extend

isl_sample.c: tab_shift_cone: use isl_tab_extend_cons instead of isl_tab_extend

Since an isl_tab is not reference counted, there is no way for
tab_shift_cone to tell its callers that the isl_tab has already
been freed, possible resulting in double frees on errors
in isl_tab_detect_equalities.  tab_shift_cone calls isl_tab_extend
which is just a wrapper around isl_tab_extend_cons turning
the appropriate interface of returning -1 on error into a bad interface
of freeing the isl_tab and returning NULL.  Call isl_tab_extend_cons
directly instead.

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