sw: update assert

After
commit 91a42a804c
Author: Xisco Fauli <xiscofauli@libreoffice.org>
Date:   Tue Aug 18 17:25:29 2026 +0200

    sw: fix crash in ModelToViewHelper::ModelToViewHelper

Change-Id: I0acdba1c0c344000c2aa131113ab9f6a0ace0049
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/210278
Tested-by: Jenkins
Reviewed-by: Xisco Fauli <xiscofauli@libreoffice.org>

sw/source/core/txtnode/modeltoviewhelper.cxx

@@ -181,7 +181,7 @@ ModelToViewHelper::ModelToViewHelper(const SwTextNode &rNode,
                 }
                 case CH_TXT_ATR_FIELDSEP:
                 {
-                    assert(startedFields.back().first->IsCoveringPosition(SwPosition(rNode, i)));
+                    assert(!startedFields.empty() && startedFields.back().first->IsCoveringPosition(SwPosition(rNode, i)));
                     if (!startedFields.empty())
                     {
                         startedFields.back().second = true;