isl_tab_pip.c: extract_sample_sequence: handle rational rows

isl_tab_pip.c: extract_sample_sequence: handle rational rows

Even though extract_sample_sequence is only called on tableaus
with an integral sample value, some of the corresponding rows
may still have a denominator different from 1.
If these denominators happen to be the same across all relevant rows,
then this does not affect the correctness because the result
is only used to check linear constraints.
However, there is no guarantee that these denominators are the same.
This problem was introduced together with the initial extract_sample_sequence
implementation in 83fb8c8 (isl_tab_basic_set_non_trivial_lexmin:
take triviality directions, Wed May 3 12:55:00 2017 +0200).
Take into account the denominators.

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