query for ibc header if trusted consensus height is zero. (cosmos#857)

relayer/processor/path_processor_internal.go

@@ -324,7 +324,8 @@ func (pp *PathProcessor) assembleMsgUpdateClient(ctx context.Context, src, dst *
 	// the latest block, we cannot send a MsgUpdateClient until another block is observed on the counterparty.
 	// If the client state height is in the past, beyond ibcHeadersToCache, then we need to query for it.
 	if !trustedConsensusHeight.EQ(clientConsensusHeight) {
-		if int64(clientConsensusHeight.RevisionHeight)-int64(trustedConsensusHeight.RevisionHeight) <= clientConsensusHeightUpdateThresholdBlocks {
+		deltaConsensusHeight := int64(clientConsensusHeight.RevisionHeight) - int64(trustedConsensusHeight.RevisionHeight)
+		if trustedConsensusHeight.RevisionHeight != 0 && deltaConsensusHeight <= clientConsensusHeightUpdateThresholdBlocks {
 			return nil, fmt.Errorf("observed client trusted height: %d does not equal latest client state height: %d",
 				trustedConsensusHeight.RevisionHeight, clientConsensusHeight.RevisionHeight)
 		}