apparmor: fix uninitialize table variable in error in unpack_trans_table

The error path has one case where *table is uninitialized, initialize
it.

Fixes: a0792e2c ("apparmor: make transition table unpack generic so it can be reused")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>