xmlsecurity: fix DrMemory warning

Presumably that was the intent?

(cherry picked from commit a933990a)

Change-Id: Ibfca90041d6913827d2f329bab448afe4d725b46
Reviewed-on: https://gerrit.libreoffice.org/20289Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>
(cherry picked from commit 3faec544)

xmlsecurity/source/xmlsec/saxhelper.cxx

@@ -137,7 +137,7 @@ SAXHelper::SAXHelper( )
     {
         xmlSAXVersion(m_pParserCtxt->sax, 1);
 
-        if( m_pParserCtxt->inputTab[0] != NULL )
+        if (m_pParserCtxt->inputTab != nullptr)
         {
             m_pParserCtxt->inputTab[0] = NULL ;
         }