Support parsing encrypted documents in XRef.indexObjects (issue 15893)

*Please note:* The reduced test-case is *not* a perfect reproduction of the original PDF document, since this one fails to open in e.g. Adobe Reader, but I do believe that it captures the most important points here.

For corrupt *and* encrypted PDF documents, it's possible that only some trailer dictionaries actually contain an /Encrypt-entry. Previously we'd could easily miss that, since we generally pick the first not obviously corrupt trailer dictionary, and the solution implemented here is to simply pre-parse all trailer dictionaries to see if there's any /Encrypt-entries.

test/pdfs/.gitignore

@@ -413,6 +413,7 @@
 !issue2128r.pdf
 !bug1703683_page2_reduced.pdf
 !issue5540.pdf
+!issue15893_reduced.pdf
 !issue5549.pdf
 !visibility_expressions.pdf
 !issue5475.pdf

test/test_manifest.json

@@ -386,6 +386,13 @@
       "lastPage": 4,
       "type": "eq"
     },
+    {  "id": "issue15893_reduced",
+       "file": "pdfs/issue15893_reduced.pdf",
+       "md5": "cf889b927f9f53d164622a99378bf39d",
+       "rounds": 1,
+       "type": "eq",
+       "password": "test"
+    },
     {  "id": "bug1727053",
        "file": "pdfs/bug1727053.pdf",
        "md5": "8ed1e52da64000f9fdcb8b732f5a58f8",