cgroups: fix -Waddress warning

cgroups: fix -Waddress warning

While in principle the pointer could overflow, GCC 12 considers this to
not be possible and issues the following warning:

  ../src/lxc/cgroups/cgfsng.c: In function ‘__cgfsng_delegate_controllers’:
  ../src/lxc/cgroups/cgfsng.c:3306:21: warning: the comparison will always evaluate as ‘true’ for the pointer operand in ‘it + 8’ must not be NULL [-Waddress]
   3306 |                 if ((it + 1) && *(it + 1))
        |                     ^

This removes the only build warning triggered when building on openSUSE.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>