tipc: Fix problem with bundled multicast message

Set the destination node and destination port fields of an outgoing
multicast message header to zero; this is necessary to ensure that
the receiving node can route the message properly if it was packed
into a bundle due to link congestion. (Previously, there was a chance
that the receiving node would send the unbundled message to a random
node & port, rather than processing the message itself.)

Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>