Wrong Maxima integral result for even exponents

[paulmasson]

As reported on this ask question, the commands

var('n')
assume(n,'integer')
integral(x^n*sin(x),x)

produce the output

1/4*(((-1)^n - 1)*gamma(n + 1, I*x) - ((-1)^n - 1)*gamma(n + 1, -I*x))*(-1)^(-1/2*n)

which is identically zero for n even. Possibly an issue with branches of exponential integrals. Possibly related to #17968.

Upstream: Not yet reported upstream; Will do shortly.

Component: symbolics

Issue created by migration from https://trac.sagemath.org/ticket/22138

[sagetrac-tmonteil]

Description changed:

--- 
+++ 
@@ -1,4 +1,4 @@
-The commands
+As reported on [this ask question](https://ask.sagemath.org/question/36185/two-ways-of-integrating-x-xnsinx-give-contradictory-results-bug/), the commands
 
 ```
 var('n')

[kcrisman]

comment:2

Don't forget to report upstream if possible.

[kcrisman]

Upstream: Not yet reported upstream; Will do shortly.