fix MM bot panic (see decred#3130 for details)

client/core/core.go

@@ -5766,23 +5766,23 @@ type MultiTradeResult struct {
 // MultiTrade is used to place multiple standing limit orders on the same
 // side of the same market simultaneously.
 func (c *Core) MultiTrade(pw []byte, form *MultiTradeForm) []*MultiTradeResult {
-	results := make([]*MultiTradeResult, len(form.Placements))
+	results := make([]*MultiTradeResult, 0, len(form.Placements))
 	reqs, err := c.prepareMultiTradeRequests(pw, form)
 	if err != nil {
-		for i := range results {
-			results[i] = &MultiTradeResult{Error: err}
+		for range form.Placements {
+			results = append(results, &MultiTradeResult{Error: err})
 		}
 		return results
 	}
 
-	for i, req := range reqs {
+	for _, req := range reqs {
 		var corder *Order
 		corder, err = c.sendTradeRequest(req)
 		if err != nil {
-			results[i] = &MultiTradeResult{Error: err}
+			results = append(results, &MultiTradeResult{Error: err})
 			continue
 		}
-		results[i] = &MultiTradeResult{Order: corder}
+		results = append(results, &MultiTradeResult{Order: corder})
 	}
 
 	return results