attach: and cgroup.c: be overly cautious

attach: and cgroup.c: be overly cautious

Realistically (as Dwight points out) it doesn't seem possible that
getline won't return at least one line in this functions, however
just to make absolutely sure we don't get a segv on free(NULL),
check line != NULL before freeing it on exit.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>