Add basic validation of the 'trailer' dictionary candidates in XRef.indexObjects (issue 9418)

This patch avoids choosing a (possible) 'trailer' dictionary that `XRef.parse` and/or the `Catalog` constructor/methods will reject anyway.
Since `XRef.indexObjects` is already parsing the entire PDF file, the extra dictionary look-ups added here shouldn't matter much. Besides, this is a fallback code-path that only applies to corrupt PDF files anyway.

test/pdfs/.gitignore

@@ -68,6 +68,7 @@
 !issue9252.pdf
 !issue9262_reduced.pdf
 !issue9291.pdf
+!issue9418.pdf
 !issue9458.pdf
 !bad-PageLabels.pdf
 !decodeACSuccessive.pdf

test/test_manifest.json

@@ -748,6 +748,13 @@
        "link": false,
        "type": "eq"
     },
+    {  "id": "issue9418",
+       "file": "pdfs/issue9418.pdf",
+       "md5": "32ecad8098acb1938539d47944ecb54b",
+       "rounds": 1,
+       "link": false,
+       "type": "eq"
+    },
     {  "id": "issue9262",
        "file": "pdfs/issue9262_reduced.pdf",
        "md5": "5347ce2d7b3866625c22e115fd90e0de",