runtime: don't crash if no p in kickoff

runtime: don't crash if no p in kickoff

    The kickoff function for g0 can be invoked without a p, for example
    from mcall(exitsyscall0) in exitsyscall after exitsyscall has cleared
    the p field. The assignment gp.param = nil will invoke a write barrier.
    If gp.param is not already nil, this will require a p. Avoid the problem
    for a specific case that is known to be OK: when the value in gp.param
    is a *g.

    Reviewed-on: https://go-review.googlesource.com/46512

From-SVN: r249595