ofz#4940 Null-dereference READ

Change-Id: I2c8c83ef6ffde5f1599814538acc36ec5beab6c8
Reviewed-on: https://gerrit.libreoffice.org/47387Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

unoxml/source/dom/element.cxx

@@ -132,7 +132,7 @@ namespace DOM
                                                                 RTL_TEXTENCODING_UTF8));
         }
 
-        const xmlChar* pPrefix = m_aNodePtr->ns ? m_aNodePtr->ns->prefix : reinterpret_cast<const xmlChar*>("");
+        const xmlChar* pPrefix = (m_aNodePtr->ns && m_aNodePtr->ns->prefix) ? m_aNodePtr->ns->prefix : reinterpret_cast<const xmlChar*>("");
         const xmlChar* pName = m_aNodePtr->name;
         sal_Int32 nElementToken=FastToken::DONTKNOW;
         if( strlen(reinterpret_cast<char const *>(pPrefix)) )