(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.

mysql-test/r/rpl_temporary.result:
  Try to have a non-SUPER user KILL a system thread.
mysql-test/t/rpl_temporary.test:
  Try to have a non-SUPER user KILL a system thread.
sql/sql_parse.cc:
  Make sure security contexts of both KILLer *and*
          KILLee are non-NULL before testing for string-equality!