build: Enable gometalinter "staticcheck" check

lib/model/rwfolder.go

@@ -494,7 +494,7 @@ nextFile:
 		select {
 		case <-f.stop:
 			// Stop processing files if the puller has been told to stop.
-			break
+			break nextFile
 		default:
 		}
 

lib/sync/sync_test.go

@@ -157,6 +157,7 @@ func TestRWMutex(t *testing.T) {
 	}()
 
 	mut.Lock()
+	_ = 1 // skip empty critical section check
 	mut.Unlock()
 
 	if len(messages) != 2 {
@@ -170,6 +171,7 @@ func TestRWMutex(t *testing.T) {
 	mut.RLock()
 	mut.RLock()
 	mut.RLock()
+	_ = 1 // skip empty critical section check
 	mut.RUnlock()
 	mut.RUnlock()
 	mut.RUnlock()