tdf#122127 Crash in FORM EDIT using drag drop to move control in dataform

regression from
    commit 2629aac3
    Date:   Tue Oct 23 09:47:10 2018 +0200
    clang-tidy performance-unnecessary-copy-init in svx

Change-Id: I4210c328145d062d8b72f09dcff3280e11c6bc36
Reviewed-on: https://gerrit.libreoffice.org/65394
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

svx/source/form/navigatortree.cxx

@@ -1000,7 +1000,7 @@ namespace svxform
         DBG_ASSERT( DND_ACTION_COPY != _nAction, "NavigatorTree::implExecuteDataTransfer: somebody changed the logics!" );
 
         // list of dragged entries
-        const ListBoxEntrySet& aDropped = _rData.selected();
+        const ListBoxEntrySet aDropped = _rData.selected();
         DBG_ASSERT(!aDropped.empty(), "NavigatorTree::implExecuteDataTransfer: no entries!");
 
         // shell and model