tipc: fix potential null pointer dereferences in some compat functions

Before calling the nla_parse_nested function, make sure the pointer to the
attribute is not null. This patch fixes several potential null pointer
dereference vulnerabilities in the tipc netlink functions.

Signed-off-by: Baozeng Ding <sploving1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>