Gossip discovery: validate dead members bug fix

When receiving a membership request, the validation
of dead peers messages was done on the top level message instead

Change-Id: I1596358993404af6b01b813e5cbf053c48ba8d31
Signed-off-by: Yacov Manevich <yacovm@il.ibm.com>

gossip/discovery/discovery_impl.go

@@ -337,7 +337,7 @@ func (d *gossipDiscoveryImpl) handleMsgFromComm(m *proto.SignedGossipMessage) {
 				d.logger.Warning("Membership response contains an invalid message from an offline peer", err)
 				return
 			}
-			if !d.crypt.ValidateAliveMsg(m) {
+			if !d.crypt.ValidateAliveMsg(dm) {
 				d.logger.Warningf("Alive message isn't authentic, someone spoofed %s's identity", dm.GetAliveMsg().Membership)
 				continue
 			}