p2p/trust: Fix nil pointer error on TrustMetric Copy() (#1819)

p2p/trust/metric.go

@@ -256,12 +256,13 @@ func (tm *TrustMetric) SetTicker(ticker MetricTicker) {
 
 // Copy returns a new trust metric with members containing the same values
 func (tm *TrustMetric) Copy() *TrustMetric {
-	tm.mtx.Lock()
-	defer tm.mtx.Unlock()
 	if tm == nil {
 		return nil
 	}
 
+	tm.mtx.Lock()
+	defer tm.mtx.Unlock()
+
 	return &TrustMetric{
 		proportionalWeight: tm.proportionalWeight,
 		integralWeight:     tm.integralWeight,

p2p/trust/metric_test.go

@@ -56,6 +56,14 @@ func TestTrustMetricConfig(t *testing.T) {
 	tm.Wait()
 }
 
+func TestTrustMetricCopyNilPointer(t *testing.T) {
+	var tm *TrustMetric
+
+	ctm := tm.Copy()
+
+	assert.Nil(t, ctm)
+}
+
 // XXX: This test fails non-deterministically
 func _TestTrustMetricStopPause(t *testing.T) {
 	// The TestTicker will provide manual control over