remove null dereference from netlink/link.h
remove null dereference from netlink/link.h

Replace a null pointer dereference with a use of the 'offsetof' macro in stddef.h.

http://lists.infradead.org/pipermail/libnl/2016-January/002085.html

Signed-off-by: Nick Lewycky <b65dea38f44fc0840ca2e84b40237dea441e418c@google.com>
Signed-off-by: Thomas Haller <16e26612e4159990a6b3691e92592e691ff74132@redhat.com>

dsahern/libnl,thom311/libnl,HolyShitMan/libnl-1,dsahern/libnl,greearb/libnl-ct,HolyShitMan/libnl-1,thom311/libnl,tklauser/libnl,dsahern/libnl,tklauser/libnl,tklauser/libnl,tklauser/libnl,HolyShitMan/libnl-1,dsahern/libnl,greearb/libnl-ct,thom311/libnl,HolyShitMan/libnl-1,greearb/libnl-ct