netlink-socket: remove local variable in do_lookup_genl_family.

'sock' is not initialized and hence should not be un-initialized
as well in the failure path.

Reported-by: Gurucharan Shetty <shettyg@nicira.com>
Signed-off-by: Nithin Raju <nithin@vmware.com>
Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>