Fix possible NRE 
Fix possible NRE 

If for some reason NUnit.Engine.Runners.ProcessRunner failed to run task, in the resultNode there is no "result". Thus OverallResult stays null and WriteSummaryReport() fails with NullReferenceException leaving user frustrated about reasons and result.

Albeit main cause of this error is rethrowing exception in ProcessRunner.cs (See Dispose method, line 255) user should not see NullReference exception and must be infromed with a more meaningfull information.
nunit/nunit-console,nunit/nunit-console,nunit/nunit-console