isl_mat_left_hermite: avoid undefined null pointer arithmetic
isl_mat_left_hermite: avoid undefined null pointer arithmetic

Technically, arithmetic on null pointers is undefined behavior,
even if only adding zero.  This may happen inside
isl_mat_left_hermite if the number of columns is zero.
Simply skip the entire computation in that case.

Signed-off-by: Michael Kruse <8300014ab702124f340a1eb40c7d70d0a8f7b3f1@meinersbur.de>
Signed-off-by: Sven Verdoolaege <235c10dd23b819f81cdc9756a251746bc184cab6@gmail.com>

Meinersbur/isl,Meinersbur/isl,Meinersbur/isl,Meinersbur/isl