drop isl_tab_extend in favor of isl_tab_extend_cons

drop isl_tab_extend in favor of isl_tab_extend_cons

Since an isl_tab is not reference counted, there may be no way for
callers of isl_tab_extend their callers that the isl_tab has already
been freed, possible resulting in double frees on errors.
isl_tab_extend 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.
Move the single remaining call to isl_tab_extend to a call
to isl_tab_extend_cons.  This does not solve any problems in itself
but should avoid future problems with isl_tab_extend.

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