bugfix: queue/Example_connectionPool data race

Part of #218

Author: oleg-jukovec

queue/example_connection_pool_test.go

@@ -185,8 +185,12 @@ func Example_connectionPool() {
 
 	// Wait for a new master instance re-identification.
 	<-h.masterUpdated
-	if h.err != nil {
-		fmt.Printf("Unable to re-identify in the pool: %s", h.err)
+	h.mutex.Lock()
+	err = h.err
+	h.mutex.Unlock()
+
+	if err != nil {
+		fmt.Printf("Unable to re-identify in the pool: %s", err)
 		return
 	}