fix: sync pool race condition (#947)

internal/tracker/seen.go

@@ -57,7 +57,11 @@ type SeenTracker struct {
 	currentIdx int
 }
 
-var pool sync.Pool
+var pool = sync.Pool{
+	New: func() interface{} {
+		return &SeenTracker{}
+	},
+}
 
 func (s *SeenTracker) reset() {
 	// Always contains a root element at index 0.
@@ -331,12 +335,6 @@ func (s *SeenTracker) checkArray(node *unstable.Node) (first bool, err error) {
 }
 
 func (s *SeenTracker) checkInlineTable(node *unstable.Node) (first bool, err error) {
-	if pool.New == nil {
-		pool.New = func() interface{} {
-			return &SeenTracker{}
-		}
-	}
-
 	s = pool.Get().(*SeenTracker)
 	s.reset()