fix nullptr/boost compile error

error: no match for ternary ‘operator?:’ in
‘((writerfilter::dmapper::DomainMapper_Impl*)this)->writerfilter::dmapper::DomainMapper_Impl::m_aFieldStack.std::stack<_Tp,
_Sequence>::empty<boost::shared_ptr<writerfilter::dmapper::FieldContext>,
std::deque<boost::shared_ptr<writerfilter::dmapper::FieldContext>,
std::allocator<boost::shared_ptr<writerfilter::dmapper::FieldContext> > > >() ?
nullptr :
((writerfilter::dmapper::DomainMapper_Impl*)this)->writerfilter::dmapper::DomainMapper_Impl::m_aFieldStack.std::stack<_Tp,
_Sequence>::top<boost::shared_ptr<writerfilter::dmapper::FieldContext>,
std::deque<boost::shared_ptr<writerfilter::dmapper::FieldContext>,
std::allocator<boost::shared_ptr<writerfilter::dmapper::FieldContext> > > >()’

Change-Id: I76c7507390bcf80f6ca722c4eedfd65c9a46952f

writerfilter/source/dmapper/DomainMapper_Impl.cxx

@@ -3361,7 +3361,9 @@ void DomainMapper_Impl::CloseFieldCommand()
     dmapper_logger->element("closeFieldCommand");
 #endif
 
-    FieldContextPtr pContext = m_aFieldStack.empty() ? nullptr : m_aFieldStack.top();
+    FieldContextPtr pContext;
+    if(!m_aFieldStack.empty())
+        pContext = m_aFieldStack.top();
     OSL_ENSURE( pContext.get(), "no field context available");
     if( pContext.get() )
     {