storage/overlay: do not write to invalid memory
storage/overlay: do not write to invalid memory

Closes #1802.

Signed-off-by: Christian Brauner <48455ab3070520a2d174545c7239d6d0fabd9a83@ubuntu.com>

terceiro/lxc,hallyn/lxc,caglar10ur/lxc,thmo/lxc,hallyn/lxc,ss1h2a3tw/lxc,terceiro/lxc,ss1h2a3tw/lxc,ss1h2a3tw/lxc,terceiro/lxc,thmo/lxc,thmo/lxc,caglar10ur/lxc,caglar10ur/lxc,hallyn/lxc