Fix the Risch Piecewise code

The code was wrong for two reasons:

- It assumed that it could set the base of the exponent equal to 0 by dividing
by the exponential term DE.t, but it really needed to set it equal to 1.

- It didn't account for the constant term (term in the polynomial of
exponentials with power 0, i in the code), which is integrated recursively,
and therefore was previously included twice.
Merge pull request #2414 from asmeurer/risch_subs

Fix the Risch Piecewise code