Fix panic when rejecting empty batch (#5524)

(cherry picked from commit 0b5811e)

internal/models/buffer.go

@@ -182,6 +182,10 @@ func (b *Buffer) Reject(batch []telegraf.Metric) {
 	b.Lock()
 	defer b.Unlock()
 
+	if len(batch) == 0 {
+		return
+	}
+
 	older := b.dist(b.first, b.batchFirst)
 	free := b.cap - b.size
 	restore := min(len(batch), free+older)
@@ -191,7 +195,8 @@ func (b *Buffer) Reject(batch []telegraf.Metric) {
 	rp := b.last
 	re := b.nextby(rp, restore)
 	b.last = re
-	for rb != rp {
+
+	for rb != rp && rp != re {
 		rp = b.prev(rp)
 		re = b.prev(re)
 

internal/models/buffer_test.go

@@ -714,3 +714,15 @@ func TestBuffer_AddOverwriteAndRejectOffset(t *testing.T) {
 	require.Equal(t, 13, reject)
 	require.Equal(t, 5, accept)
 }
+
+func TestBuffer_RejectEmptyBatch(t *testing.T) {
+	b := setup(NewBuffer("test", 5))
+	batch := b.Batch(2)
+	b.Add(MetricTime(1))
+	b.Reject(batch)
+	b.Add(MetricTime(2))
+	batch = b.Batch(2)
+	for _, m := range batch {
+		require.NotNil(t, m)
+	}
+}