tipc: do sanity check payload of a netlink message

When we initialize nlmsghdr with no payload inside tipc_nl_compat_dumpit()
the parsing function returns -EINVAL. We fix it by making the parsing call
conditional.

Acked-by: Jon Maloy <jmaloy@redhat.com>
Signed-off-by: Hoang Le <hoang.h.le@dektech.com.au>
Link: https://lore.kernel.org/r/20201215033151.76139-1-hoang.h.le@dektech.com.au


Signed-off-by: Jakub Kicinski <kuba@kernel.org>