isl_mat_variable_compression: only set *T2 to NULL on error

isl_mat_variable_compression: only set *T2 to NULL on error

When isl_mat_variable_compression figures out that the input equalities
do not allow for any integer solutions, it returns a zero-column matrix
and clears the inverse transformation, assigning it the value NULL.
The inverse transformation is useless in such cases, but some callers
of isl_mat_variable_compression treat a NULL value for the inverse
transformation as an error.  This happens in particular in
isl_basic_map_reduce_coefficients and also in normalize_divs in
isl_map_simplify.c.  This can result in a spurious coalesce operation
failure.  The other callers test for a zero-column transformation first,
but free the returned inverse transformation anyway.

Since the inverse transformation cannot be used in case of a zero-column
transformation, it would not hurt to simply return the current value
of the inverse transformation, whatever it is.
Still, it seems cleaner to return a matching zero-row matrix instead,
which is what this commit does.

Reported-by: antoine.morvan.pro@gmail.com
Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>