guard against missing RootDocument

Change-Id: I1c6b58b58ab489a17419dbf7cd4ecec63359b7f3
(cherry picked from commit 0f700d5b)
Reviewed-on: https://gerrit.libreoffice.org/20515Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

lotuswordpro/source/filter/lwpdoc.cxx

@@ -634,8 +634,8 @@ LwpDocument* LwpDocument::GetPreviousDivision()
  LwpDocument* LwpDocument::GetLastDivisionThatHasEndnote()
 {
     LwpDocument* pRoot = GetRootDocument();
-    LwpDocument *pLastDoc = pRoot->GetLastDivisionWithContents();
-    while(pLastDoc)
+    LwpDocument *pLastDoc = pRoot ? pRoot->GetLastDivisionWithContents() : nullptr;
+    while (pLastDoc)
     {
         if(pLastDoc->GetEnSuperTableLayout())
             return pLastDoc;