cgfsng: fix cgroup attach cgroup creation

cgfsng: fix cgroup attach cgroup creation

\e[01m\e[Kcgroups/cgfsng.c:\e[m\e[K In function ‘\e[01m\e[Kcgroup_attach_leaf.constprop\e[m\e[K’:
\e[01m\e[Kcgroups/cgfsng.c:2221:10:\e[m\e[K \e[01;31m\e[Kerror: \e[m\e[Kwriting 1 byte into a region of size 0 [\e[01;31m\e[K-Werror=stringop-overflow=\e[m\e[K]
 2221 |   \e[01;31m\e[K*slash = '\0'\e[m\e[K;
      |   \e[01;31m\e[K~~~~~~~^~~~~~\e[m\e[K
\e[01m\e[Kcgroups/cgfsng.c:2213:8:\e[m\e[K \e[01;36m\e[Knote: \e[m\e[Kat offset -13 to object ‘\e[01m\e[Kattach_cgroup\e[m\e[K’ with size 23 declared here
 2213 |   char \e[01;36m\e[Kattach_cgroup\e[m\e[K[STRLITERALLEN(".lxc-1000/cgroup.procs") + 1];
      |        \e[01;36m\e[K^~~~~~~~~~~~~\e[m\e[K
\e[01m\e[Kcgroups/cgfsng.c:2229:10:\e[m\e[K \e[01;31m\e[Kerror: \e[m\e[Kwriting 1 byte into a region of size 0 [\e[01;31m\e[K-Werror=stringop-overflow=\e[m\e[K]
 2229 |   \e[01;31m\e[K*slash = '/'\e[m\e[K;
      |   \e[01;31m\e[K~~~~~~~^~~~~\e[m\e[K
\e[01m\e[Kcgroups/cgfsng.c:2213:8:\e[m\e[K \e[01;36m\e[Knote: \e[m\e[Kat offset -13 to object ‘\e[01m\e[Kattach_cgroup\e[m\e[K’ with size 23 declared here
 2213 |   char \e[01;36m\e[Kattach_cgroup\e[m\e[K[STRLITERALLEN(".lxc-1000/cgroup.procs") + 1];
      |        \e[01;36m\e[K^~~~~~~~~~~~~\e[m\e[K
\e[01m\e[Kcgroups/cgfsng.c:2229:10:\e[m\e[K \e[01;31m\e[Kerror: \e[m\e[Kwriting 1 byte into a region of size 0 [\e[01;31m\e[K-Werror=stringop-overflow=\e[m\e[K]
 2229 |   \e[01;31m\e[K*slash = '/'\e[m\e[K;
      |   \e[01;31m\e[K~~~~~~~^~~~~\e[m\e[K
\e[01m\e[Kcgroups/cgfsng.c:2213:8:\e[m\e[K \e[01;36m\e[Knote: \e[m\e[Kat offset -13 to object ‘\e[01m\e[Kattach_cgroup\e[m\e[K’ with size 23 declared here
 2213 |   char \e[01;36m\e[Kattach_cgroup\e[m\e[K[STRLITERALLEN(".lxc-1000/cgroup.procs") + 1];
      |        \e[01;36m\e[K^~~~~~~~~~~~~\e[m\e[K

Link: https://launchpadlibrarian.net/494354168/buildlog_ubuntu-groovy-armhf.lxc_1%3A4.0.4-0ubuntu1_BUILDING.txt.gz
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>