When parsing corrupt documents without any trailer-dictionary, fallback to the "top"-dictionary (issue 14269)

There's obviously no guarantee that this will work in general, if the document is sufficiently corrupt, but it should hopefully be better than just throwing `InvalidPDFException` as currently happens.

Please note that, as is often the case with corrupt documents, it's somewhat difficult to know if we're rendering the document "correctly" with this patch[1]. In this case even Adobe Reader cannot open the document, which is always a good sign that it's *really* corrupt, however we're at least able to render *something* with this patch.

---
[1] Whatever "correct" even means when dealing with corrupt PDF documents, where often times different PDF viewers won't agree completely.

src/core/xref.js

@@ -590,6 +590,10 @@ class XRef {
     if (trailerDict) {
       return trailerDict;
     }
+    // No trailer dictionary found, taking the "top"-dictionary (if exists).
+    if (this.topDict) {
+      return this.topDict;
+    }
     // nothing helps
     throw new InvalidPDFException("Invalid PDF structure.");
   }
@@ -680,6 +684,8 @@ class XRef {
         throw e;
       }
       info("(while reading XRef): " + e);
+
+      this.startXRefQueue.shift();
     }
 
     if (recoveryMode) {