Belts and braces for crash in null pointer dereference

Change-Id: I17fa205f16929b913084c8eee320a73416b1eda0
(cherry picked from commit d34a1590)
Reviewed-on: https://gerrit.libreoffice.org/3605Reviewed-by: Michael Meeks <michael.meeks@suse.com>
Tested-by: Michael Meeks <michael.meeks@suse.com>

sw/source/core/unocore/unotext.cxx

@@ -1626,7 +1626,8 @@ throw (lang::IllegalArgumentException, uno::RuntimeException)
             SwTableNode * pStartTableNode(pStartStartNode->FindTableNode());
             // Is it the same table start node than the end?
             SwTableNode *const pEndStartTableNode(pEndStartNode->FindTableNode());
-            while (pEndStartTableNode->GetIndex() < pStartTableNode->GetIndex())
+            while (pEndStartTableNode && pStartTableNode &&
+                   pEndStartTableNode->GetIndex() < pStartTableNode->GetIndex())
             {
                 SwStartNode* pStartStartTableNode = pStartTableNode->StartOfSectionNode();
                 pStartTableNode = pStartStartTableNode->FindTableNode();