Treat loop in xref tables as damage (fixes #192)

Prior to this fix, if there was a loop detected in following /Prev
pointers in xref streams/tables, it would cause qpdf to lose data.
Note that this condition causes many PDF readers to hang or fail.

1 files changed, 3 insertions, 1 deletions

diff --git a/libqpdf/QPDF.cc b/libqpdf/QPDF.cc
index 9e30d6db..90e90813 100644
--- a/libqpdf/QPDF.cc
+++ b/libqpdf/QPDF.cc
@@ -532,7 +532,9 @@ QPDF::read_xref(qpdf_offset_t xref_offset)
 	}
         if (visited.count(xref_offset) != 0)
         {
-            xref_offset = 0;
+            QTC::TC("qpdf", "QPDF xref loop");
+            throw QPDFExc(qpdf_e_damaged_pdf, this->m->file->getName(), "", 0,
+                          "loop detected following xref tables");
         }
     }