Remove extra memset from __nlmsg_alloc

A few lines above nm->nm_nlh is already allocated with calloc, the
allocated memory has at least sizeof(struct nlmsghdr).

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Merge pull request #50 from olafhering/olh_memset__nlmsg_alloc

Remove extra memset from __nlmsg_alloc