Prevent an infinite loop in XRef.readXRef by keeping track of already parsed tables (bug 1393476)

With this patch, not only is the infinite loop prevented, but we're also able to actually render the file (which e.g. Adobe Reader isn't able to).

Fixes https://bugzilla.mozilla.org/show_bug.cgi?id=1393476.

test/pdfs/bug1393476.pdf.link

@@ -0,0 +1 @@
+https://bugzilla.mozilla.org/attachment.cgi?id=8900754

test/test_manifest.json

@@ -820,6 +820,13 @@
        "link": false,
        "type": "eq"
     },
+    {  "id": "bug1393476",
+       "file": "pdfs/bug1393476.pdf",
+       "md5": "163ee8727c77f27ee651eec777bb20a9",
+       "rounds": 1,
+       "link": true,
+       "type": "eq"
+    },
     {  "id": "bug1252420",
        "file": "pdfs/bug1252420.pdf",
        "md5": "f21c911b9b655972b06ef782a1fa6a17",