Fix the link for getting the incident stacktrace after the process instance is completed.

Steps to reproduce:

• create a process instance with an incident
• check the stacktrace of the incident
  • runtime/history have the same link:
    http://localhost:8080/camunda/api/engine/engine/default/job/7979011f-33e5-11e7-9b72-00aa004d0001/stacktrace
• complete the process instance
• check the stacktrace of the incident again
  • same link like above is used and an exception is thrown (see the related support issue for the stacktrace)