LibPDF: Don't check for `startxref` after trailer dict

Several files have a comment after the trailer dict and the
`startxref` after it.

We really should add a consume_whitespace_and_comments() function
and call that in most places we currently call consume_whitespace().

But in this case, for non-linearized files, we first jump to the
end of the file, read `startxref`, then jump to `xref` from the
offset there, and then read the trailer after the `xref`,
only to read `startxref` again. So we can just not do that.

(For linearized files, we now completely ignore `startxref`.
But we don't use the data in `startxref` in linearized files
anyways, so it's fine to not read it there too.)

Reduces number of crashes on 300 random PDFs from the web (the first 300
from 0000.zip from
https://pdfa.org/new-large-scale-pdf-corpus-now-publicly-available/)
from 25 (8%) to 23 (7%).

Userland/Libraries/LibPDF/DocumentParser.cpp

@@ -525,21 +525,7 @@ PDFErrorOr<NonnullRefPtr<DictObject>> DocumentParser::parse_file_trailer()
         return error("Expected \"trailer\" keyword");
     m_reader.move_by(7);
     m_reader.consume_whitespace();
-    auto dict = TRY(parse_dict());
-
-    if (!m_reader.matches("startxref"))
-        return error("Expected \"startxref\"");
-    m_reader.move_by(9);
-    m_reader.consume_whitespace();
-
-    m_reader.move_until([&](auto) { return m_reader.matches_eol(); });
-    VERIFY(m_reader.consume_eol());
-    if (!m_reader.matches("%%EOF"))
-        return error("Expected \"%%EOF\"");
-
-    m_reader.move_by(5);
-    m_reader.consume_whitespace();
-    return dict;
+    return parse_dict();
 }
 
 PDFErrorOr<Value> DocumentParser::parse_compressed_object_with_index(u32 index)