tipc: remove pointless check for NULL prior to kfree

It is acceptable to call kfree() with NULL, so these checks are not
serving any useful purpose.

Signed-off-by: Allan Stephens <Allan.Stephens@windriver.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>