tipc: Optimize tipc_node_has_active_links()

Eliminate unnecessary checking for null node pointer and redundant
check of second active link array entry.

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>