risch: factor out common expression.
risch: Fix WrongResult for x*exp(-k*x).

Fix issue 3886: Incorrect Piecewise result from risch_integrate
http://code.google.com/p/sympy/issues/detail?id=3886
Merge pull request #2170 from jrioux/integrals

Fix WrongResult for risch_integrate(x*exp(-k*x), x)