UPSTREAM: selinux: ensure the policy has been loaded before reading the sidtab stats

Check to make sure we have loaded a policy before we query the
sidtab's hash stats.  Failure to do so could result in a kernel
panic/oops due to a dereferenced NULL pointer.

Fixes: 66f8e2f03c02 ("selinux: sidtab reverse lookup hash table")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>
(cherry picked from commit 15b590a81fcdd44ddcb4810f2a6334df8b6ca512)
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I7e18ac17ee6c89d2a63caf02efcfb597f21f2c54