cid#1606825 Data race condition

Change-Id: If2a0385a0b9d7d09c05cacc9bbc1034bd14f53b3
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/173947
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolan.mcnamara@collabora.com>

toolkit/source/controls/grid/defaultgriddatamodel.cxx

@@ -496,6 +496,7 @@ private:
 
     Reference< css::util::XCloneable > SAL_CALL DefaultGridDataModel::createClone(  )
     {
+        std::unique_lock aGuard(m_aMutex);
         return new DefaultGridDataModel( *this );
     }