Qt 4.4 QtConcurrent and libthread_xu

[Matthew Dillon]

    Ultimately it comes down to what does the standard intend?  

    In my years of programming I've found that it is generally better to
    enforce the stated intention by crashing the program (or allowing it
    to crash) rather then returning an error code.  It results in far better
    code quality from the people using the API.  Unless the standard
    specifically says that NULL is ok, then the problem is with the Qt
    library and not with the threads library.

    That said, from an administrative point of view I want to have maximum
    compatibility between libthread_xu and libpthread, because we aren't
    really in the business of policing other people's work.

    So I am willing to commit the change, even if the standard doesn't allow
    NULL.

					    -Matt