Power operator is broken for −1^b when b < 0

[verement]

The following change to the numbers.c function do_power() is erroneous:

-               ans.v.num = (b % 2 == 0 ? 1 : -1);
+               ans.v.num = (b & 1) ? 1 : -1;

As a result, negative powers of −1 have the wrong sign.