cgroup: initialize controllers buffer to 0s (#106)

Initialize controllers buffer to 0s in case there are no cgroups
controllers and reading "cgroup.controllers" results in 0 bytes.

Without this, the controllers buffer contents is undefined and then
later on we attempt to strtok on the undefined buffer to split it
on spaces which can yield undefined results.

cgroup.c

@@ -282,7 +282,7 @@ void cgroup_start_cleaner(int parentfd, const char *name)
 
 void cgroup_enable_controllers(int cgroupfd)
 {
-	char controllers[BUFSIZ];
+	char controllers[BUFSIZ] = {0};
 	int cfd = openat(cgroupfd, "cgroup.controllers", O_RDONLY, 0);
 	if (cfd == -1) {
 		err(1, "cgroup_enable_controllers: open cgroup.controllers");