d2d1: Fix out-of-bound array access.
d2d_fp_fast_expansion_sum_zeroelim contains many instances of this pattern:
if (a_idx < a_len)
{
use(a_curr);
a_curr = a[++a_idx];
}
Here if a_idx is already a_len - 1, a[++a_idx] reads past the last element of a.