powerpc/powerpc/vm_machdep.c:cpu_fork(): tf used uninitialized

[Tad Hunt <tad%entrisphere.com@localhost>]

``tf'' will be used uninitialized in cpu_fork(), (if ``stack != NULL'').

I haven't looked at how cpu_fork() is used, so I can't provide a bugfix.
I currently panic() if stack != NULL until I figure out what is supposed
to happen.

-Tad