Set a reasonable fallback for get_rundir

If get_rundir can't find XDG_RUNTIME_DIR in the environment, it'll
attempt to build a path using ~/.cache/lxc/run/. Should that fail
because of missing $HOME in the environment, it'll then return NULL an
all callers will fail in that case.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>