utils: fix lxc_mount_proc_if_needed()
utils: fix lxc_mount_proc_if_needed()

- check for buffer overflow
- only call INFO() after we ensured that readlink() was successful
- simplify logic

Reported-by: 60826da52354ce2aab1943d74c9d4b3cc35a2a71@web.de
Signed-off-by: Christian Brauner <48455ab3070520a2d174545c7239d6d0fabd9a83@ubuntu.com>

aeris/lxc,LynxChaus/lxc,terceiro/lxc,terceiro/lxc,caglar10ur/lxc,thmo/lxc,federicobriata/lxc,thmo/lxc,federicobriata/lxc,federicobriata/lxc,caglar10ur/lxc,hallyn/lxc,aeris/lxc,ss1h2a3tw/lxc,ss1h2a3tw/lxc,thmo/lxc,LynxChaus/lxc,terceiro/lxc,aeris/lxc,hallyn/lxc,hallyn/lxc,ss1h2a3tw/lxc,LynxChaus/lxc,caglar10ur/lxc