AppArmor: Fix underflow in xindex calculation

If the xindex value stored in the accept tables is 0, the extraction of
that value will result in an underflow (0 - 4).

In properly compiled policy this should not happen for file rules but
it may be possible for other rule types in the future.

To exploit this underflow a user would have to be able to load a corrupt
policy, which requires CAP_MAC_ADMIN, overwrite system policy in kernel
memory or know of a compiler error resulting in the flaw being present
for loaded policy (no such flaw is known at this time).

Signed-off-by: John Johansen <john.johansen@canonical.com>
Acked-by: Kees Cook <kees@ubuntu.com>