Avoid directory traversal when extracting archives

Avoid directory traversal when extracting archives by skipping over leading
slashes and any prefix containing ".." components.

Patch authored by Raphael Geissert.

Source: http://sources.debian.net/data/main/libt/libtar/1.2.20-4/debian/patches/CVE-2013-4420.patch
Reference: https://bugs.debian.org/73186
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>