Prevent destruction of shared null objects

#863 uses a single null object for nulls that were previously implicit. In
certain circumstances this shared null object gets destroyed (i.e changed
to a QPDF_Destroyed object) when a QPDF object is destroyed.

Modify the QPDF destructor so that null objects get disconnected from the
dying QPDF object but not destroyed to prevent this from happening.

1 files changed, 3 insertions, 2 deletions

diff --git a/libqpdf/QPDF.cc b/libqpdf/QPDF.cc
index 83944be4..c7cc1370 100644
--- a/libqpdf/QPDF.cc
+++ b/libqpdf/QPDF.cc
@@ -247,10 +247,11 @@ QPDF::~QPDF()
     // but we'll explicitly clear the xref table anyway just to
     // prevent any possibility of resolve() succeeding.
     this->m->xref_table.clear();
-    auto null_obj = QPDF_Null::create();
     for (auto const& iter: this->m->obj_cache) {
         iter.second.object->disconnect();
-        iter.second.object->destroy();
+        if (iter.second.object->getTypeCode() != ::ot_null) {
+            iter.second.object->destroy();
+        }
     }
 }