writerfilter: make sure the redline stack always has at least one element

One element is pushed to that stack in the constructor, so calling top()
on it always OK, as long as we check during pop() that the stack won't
be empty.

Change-Id: I010bd77897ab2a3684e7d9c5c0450e7bda626fc0

writerfilter/source/dmapper/DomainMapper_Impl.cxx

@@ -1635,6 +1635,11 @@ void DomainMapper_Impl::PopFootOrEndnote()
     if (!m_aTextAppendStack.empty())
         m_aTextAppendStack.pop();
 
+    if (m_aRedlines.size() == 1)
+    {
+        SAL_WARN("writerfilter", "PopFootOrEndnote() is called without PushFootOrEndnote()?");
+        return;
+    }
     m_aRedlines.pop();
 }