do not crash when opening rtf file with unclosed field group

It still fails to load the file (General input/output error), because
RTFDocumentImpl::resolve() throws io::UnexpectedEOFException, but that
is a bit better than a crash .-)

Example of such a file can be found in fdo#49666 or rhbz#819304 .

Change-Id: I I1e575bf8e3194afe063297a4daf3e98eaaa510cc

writerfilter/source/dmapper/DomainMapper_Impl.cxx

@@ -3137,6 +3137,9 @@ void DomainMapper_Impl::PopFieldContext()
     dmapper_logger->element("popFieldContext");
 #endif
 
+    if (m_aFieldStack.empty())
+        return;
+
     FieldContextPtr pContext = m_aFieldStack.top();
     OSL_ENSURE( pContext.get(), "no field context available");
     if( pContext.get() )