apparmor: fix oops in profile_unpack() when policy_db is not present

BugLink: http://bugs.launchpad.net/bugs/1592547

If unpack_dfa() returns NULL due to the dfa not being present,
profile_unpack() is not checking if the dfa is not present (NULL).

Signed-off-by: John Johansen <john.johansen@canonical.com>