winspec_spe: fix potential out-of-bound read, thx to Coverity
winspec_spe: fix potential out-of-bound read, thx to Coverity

from impressive coverity report:

10. cond_at_most: Checking j <= calib->polynom_order implies that j has
the value which may be up to 6 on the true branch.
161            for (int j = 0; j <= calib->polynom_order; ++j)

CID 23251 (#1 of 1): Out-of-bounds read (OVERRUN)
11. overrun-local: Overrunning array calib->polynom_coeff of 6 8-byte
elements at element index 6 (byte offset 48) using index j (which
evaluates to 6).
162                x += calib->polynom_coeff[j] * pow(i + 1., double(j));

wojdyr/xylib,wojdyr/xylib,wojdyr/xylib