forcepoint #16: fix heap-use-after-free

PDFDocument::Tokenize() in the aKeyword == "obj" case allocates a
PDFObjectElement, stores it as an owning pointer inside rElements, and
also stores two non-owning references to it in m_aOffsetObjects and
m_aIDObjects. So make sure those 2 other containers are also cleared
then elements go away.

LO_TRACE="valgrind" bin/run pdfverify <sample>

doesn't report errors anymore after the fix.

Change-Id: Ie103de3e24a1080257a79e53b994e8536a9597bc
Reviewed-on: https://gerrit.libreoffice.org/50627Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>

vcl/source/filter/ipdf/pdfdocument.cxx

@@ -1260,8 +1260,10 @@ bool PDFDocument::Read(SvStream& rStream)
         if (pPrev)
             nStartXRef = pPrev->GetValue();
 
-        // Reset state, except object offsets and the edit buffer.
+        // Reset state, except the edit buffer.
         m_aElements.clear();
+        m_aOffsetObjects.clear();
+        m_aIDObjects.clear();
         m_aStartXRefs.clear();
         m_aEOFs.clear();
         m_pTrailer = nullptr;

xmlsecurity/qa/unit/pdfsigning/pdfsigning.cxx

@@ -452,6 +452,8 @@ void PDFSigningTest::testTokenize()
         "tdf107149.pdf",
         // Nested parentheses were not handled.
         "tdf114460.pdf",
+        // Valgrind was unhappy about this.
+        "forcepoint16.pdf",
     };
 
     for (const auto& rName : aNames)