[birt-dev]CheckIn: Fix unit test error

["Wei Zhao \(Shang\)" <wei.zhao@xxxxxxxxxxx>]

Title: [birt-dev]CheckIn: Fix unit test error

- Summary:
In ReportEngine.destroy(), should check the reportEngineHelper is null or not before calling its stopLogging( ) method.

- Bugzilla Bug (s) Resolved:
NONE

- Description:
In ReportEngine.destroy(), should check the reportEngineHelper is null or not before calling its stopLogging( ) method.

- Tests Description  :
Unit Test

- Notes to Build Team:

- Notes to Developers:

- Notes to QA:  
 
- Notes to Documentation:  

- Files Edited:
"/org.eclipse.birt.report.engine/src/org/eclipse/birt/report/engine/api/impl/ReportEngine.java"

- Files Added:

- Files Deleted

-Wei Zhao(Shanghai)