Check the top-level /Pages dictionary when finding the trailer in XRef.indexObjects (issue 12402)

In addition to the existing /Root and /Pages validation, also check that the /Pages-entry actually is a dictionary and that it has a valid /Count-entry.
This way we can avoid picking a trailer candidate which e.g. the `Catalog.numPages` getter will just end up rejecting, thus breaking PDF document loading completely.

src/core/obj.js

@@ -1836,14 +1836,13 @@ var XRef = (function XRefClosure() {
         }
       }
       // reading XRef streams
-      var i, ii;
-      for (i = 0, ii = xrefStms.length; i < ii; ++i) {
+      for (let i = 0, ii = xrefStms.length; i < ii; ++i) {
         this.startXRefQueue.push(xrefStms[i]);
         this.readXRef(/* recoveryMode */ true);
       }
       // finding main trailer
       let trailerDict;
-      for (i = 0, ii = trailers.length; i < ii; ++i) {
+      for (let i = 0, ii = trailers.length; i < ii; ++i) {
         stream.pos = trailers[i];
         const parser = new Parser({
           lexer: new Lexer(stream),
@@ -1861,18 +1860,26 @@ var XRef = (function XRefClosure() {
           continue;
         }
         // Do some basic validation of the trailer/root dictionary candidate.
-        let rootDict;
         try {
-          rootDict = dict.get("Root");
+          const rootDict = dict.get("Root");
+          if (!(rootDict instanceof Dict)) {
+            continue;
+          }
+          const pagesDict = rootDict.get("Pages");
+          if (!(pagesDict instanceof Dict)) {
+            continue;
+          }
+          const pagesCount = pagesDict.get("Count");
+          if (!Number.isInteger(pagesCount)) {
+            continue;
+          }
+          // The top-level /Pages dictionary isn't obviously corrupt.
         } catch (ex) {
           if (ex instanceof MissingDataException) {
             throw ex;
           }
           continue;
         }
-        if (!isDict(rootDict) || !rootDict.has("Pages")) {
-          continue;
-        }
         // taking the first one with 'ID'
         if (dict.has("ID")) {
           return dict;

test/pdfs/issue12402.pdf.link

@@ -0,0 +1 @@
+https://github.com/mozilla/pdf.js/files/5597129/issue12402.pdf

test/test_manifest.json

@@ -842,6 +842,15 @@
        "link": false,
        "type": "eq"
     },
+    {  "id": "issue12402",
+       "file": "pdfs/issue12402.pdf",
+       "md5": "70031cf610e24cc7164fb6ecd6980c8e",
+       "rounds": 1,
+       "link": true,
+       "firstPage": 8,
+       "lastPage": 8,
+       "type": "eq"
+    },
     {  "id": "issue10004",
        "file": "pdfs/issue10004.pdf",
        "md5": "64d1853060cefe3be50e5c4617dd0505",