tipc: fix info leak from kernel tipc_event

We initialize a struct tipc_event allocated on the kernel stack to
zero to avert info leak to user space.

Reported-by:  <syzbot+057458894bc8cada4dee@syzkaller.appspotmail.com>
Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>