[TIPC]: Fix simple sparse warnings

Tried to run the new tipc stack through sparse.
Following patch fixes all cases where 0 was used
as replacement of NULL.
Use NULL to document this is a pointer and to silence sparse.

This brough sparse warning count down with 127 to 24 warnings.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Per Liden <per.liden@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/tipc/bcast.c b/net/tipc/bcast.c
index a7b04f3..a926cec 100644
--- a/net/tipc/bcast.c
+++ b/net/tipc/bcast.c
@@ -271,7 +271,7 @@
 		msg_set_bcgap_to(msg, n_ptr->bclink.gap_to);
 		msg_set_bcast_tag(msg, tipc_own_tag);
 
-		if (tipc_bearer_send(&bcbearer->bearer, buf, 0)) {
+		if (tipc_bearer_send(&bcbearer->bearer, buf, NULL)) {
 			bcl->stats.sent_nacks++;
 			buf_discard(buf);
 		} else {