AppArmor: Add mising end of structure test to caps unpacking

The unpacking of struct capsx is missing a check for the end of the
caps structure.  This can lead to unpack failures depending on what else
is packed into the policy file being unpacked.

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