cid#1607743 Data race condition

Change-Id: Ic7f4744fdb1cf43f6bcf6c66752f37be25c06cb6
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/178494
Reviewed-by: Caolán McNamara <caolan.mcnamara@collabora.com>
Tested-by: Jenkins

ucb/source/sorter/sortresult.cxx

@@ -1169,6 +1169,9 @@ void SortedResultSet::PropertyChangedImpl(std::unique_lock<std::mutex>& rGuard,
 
 void SortedResultSet::CopyData( SortedResultSet *pSource )
 {
+    std::unique_lock aGuard(maMutex);
+    std::unique_lock aSrcGuard(pSource->maMutex);
+
     const SortedEntryList& rSrcS2O = pSource->maS2O;
 
     sal_IntPtr i, nCount;