isl_input.c: accept_affine_factor: protect against out-of-bounds accesses
isl_input.c: accept_affine_factor: protect against out-of-bounds accesses

In principle, the position should always be valid,
but this may not always be the case in the current code base.
Call isl_vec_set_element_si to set the coefficient,
which internally performs out-of-bounds checking.

Signed-off-by: Sven Verdoolaege <dd860110a62b19214c4ee03aec0abffecb4e86b8@cerebras.net>

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