Fundmanager shouldn't try to add zero

chain/market/fundmgr.go

@@ -2,6 +2,7 @@ package market
 
 import (
 	"context"
+	"github.com/filecoin-project/specs-actors/actors/abi/big"
 	"sync"
 
 	"github.com/filecoin-project/specs-actors/actors/builtin"
@@ -52,13 +53,16 @@ func (fm *FundMgr) EnsureAvailable(ctx context.Context, addr, wallet address.Add
 	if avail.LessThan(types.NewInt(0)) {
 		// TODO: some rules around adding more to avoid doing stuff on-chain
 		//  all the time
-		toAdd = types.BigSub(toAdd, avail)
+		toAdd = avail.Neg()
 		avail = types.NewInt(0)
 	}
 	fm.available[addr] = avail
 
 	fm.lk.Unlock()
 
+	if toAdd.LessThanEqual(big.Zero()) {
+		return cid.Undef, nil
+	} else {
 		var err error
 		params, err := actors.SerializeParams(&addr)
 		if err != nil {
@@ -78,3 +82,4 @@ func (fm *FundMgr) EnsureAvailable(ctx context.Context, addr, wallet address.Add
 
 		return smsg.Cid(), nil
 	}
+}