[TIPC]: Remove redundant NULL check when discarding buffers

This patch eliminates a null pointer check when discarding a
TIPC message buffer, since kfree_skb() already handles this
situation.

Acknowledgements to Florian Westphal (fw@strlen.de> for
suggesting this enhancement.

Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>