(Pushing for Azundris)
      
We allow security-contexts with NULL users (for
system-threads and for unauthenticated users).
If a non-SUPER-user tried to KILL such a thread,
we tried to compare the user-fields to see whether
they owned that thread. Comparing against NULL was
not a good idea.
      
If KILLer does not have SUPER-privilege, we
specifically check whether both KILLer and KILLee
have a non-NULL user before testing for string-
equality. If either is NULL, we reject the KILL.