fix: Display captions with forward slashes (#3556)

Closes #3555 .

Author: bcupac

lib/text/vtt_text_parser.js

@@ -436,7 +436,9 @@ shaka.text.VttTextParser = class {
         }
         const tagEnd = payload.substring(i + 1, end);
         const tagStart = names.pop();
-        if (tagStart === tagEnd) {
+        if (!tagEnd || !tagStart) {
+          return payload;
+        } else if (tagStart === tagEnd) {
           newPayload += '/' + tagEnd + '>';
           i += tagEnd.length + 1;
         } else {

test/text/vtt_text_parser_unit.js

@@ -924,6 +924,18 @@ describe('VttTextParser', () => {
             payload: '<c.lime><b>Parse fail 2</c></b>',
             nestedCues: [],
           },
+          {
+            startTime: 100,
+            endTime: 110,
+            payload: '<c.lime>forward slash 1/2 in text</c>',
+            nestedCues: [],
+          },
+          {
+            startTime: 110,
+            endTime: 120,
+            payload: '<c.lime>less or more < > in text</c>',
+            nestedCues: [],
+          },
         ],
         'WEBVTT\n\n' +
         '00:00:20.000 --> 00:00:40.000\n' +
@@ -939,7 +951,11 @@ describe('VttTextParser', () => {
         '00:01:20.000 --> 00:01:30.000\n' +
         '<b><c.lime>Parse fail 1</b></c>\n\n' +
         '00:01:30.000 --> 00:01:40.000\n' +
-        '<c.lime><b>Parse fail 2</c></b>',
+        '<c.lime><b>Parse fail 2</c></b>\n\n' +
+        '00:01:40.000 --> 00:01:50.000\n' +
+        '<c.lime>forward slash 1/2 in text</c>\n\n' +
+        '00:01:50.000 --> 00:02:00.000\n' +
+        '<c.lime>less or more < > in text</c>',
         {periodStart: 0, segmentStart: 0, segmentEnd: 0});
   });