eth/tracers: fix

eth/tracers/native/call.go

@@ -19,7 +19,7 @@ func init() {
 type CallFrame struct {
 	Type    string      `json:"type"`
 	From    string      `json:"from"`
-	To      string      `json:"to"`
+	To      string      `json:"to,omitempty"`
 	Value   string      `json:"value,omitempty"`
 	Gas     string      `json:"gas"`
 	GasUsed string      `json:"gasUsed"`
@@ -82,7 +82,9 @@ func (t *CallTracer) CaptureEnter(typ vm.OpCode, from common.Address, to common.
 
 func (t *CallTracer) CaptureExit(output []byte, gasUsed uint64, err error) {
 	size := len(t.callstack)
-	if size > 1 {
+	if size <= 1 {
+		return
+	}
 	// pop call
 	call := t.callstack[size-1]
 	t.callstack = t.callstack[:size-1]
@@ -98,7 +100,6 @@ func (t *CallTracer) CaptureExit(output []byte, gasUsed uint64, err error) {
 		}
 	}
 	t.callstack[size-1].Calls = append(t.callstack[size-1].Calls, call)
-	}
 }
 
 func (t *CallTracer) GetResult() (json.RawMessage, error) {