[TIPC] Fix for NULL pointer dereference

This fixes a bug spotted by the coverity checker, bug id #366. If
(mod(seqno - prev) != 1) we set buf to NULL, dereference it in the for
case, and set it to whatever value happes to be at adress 0+next, if it
happens to be non-zero, we even stay in the loop. It seems that the author
intended to break there.

Signed-off-by: Eric Sesterhenn <snakebyte@gmx.de>
Signed-off-by: Per Liden <per.liden@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>