if iwpriv turbo is set before first beacon update it will segfault because the update...

if iwpriv turbo is set before first beacon update it will segfault because the update for dturbo comes
before the allocation and initial configuration.  Since there shouldn't be any dturbo switches at this point, some null checks prevent the problem.  I'm not sure where the code should be moved to, but probably further down in the function.

git-svn-id: http://madwifi-project.org/svn/madwifi/trunk@3498 0192ed92-7a03-0410-a25b-9323aeb14dbd