解决偶发的漏最后一行字幕的BUG

Author: nilaoda

src/N_m3u8DL-RE.Common/Entity/WebVttSub.cs

@@ -50,6 +50,7 @@ public static WebVttSub Parse(string text, long BaseTimestamp = 0L)
             if (!text.Trim().StartsWith("WEBVTT"))
                 throw new Exception("Bad vtt!");
 
+            text += Environment.NewLine;
 
             var webSub = new WebVttSub();
             var needPayload = false;