proper theory naming after join (reset due to merge_data);

--- a/src/Pure/theory.ML	Thu Jul 04 12:31:24 2019 +0200
+++ b/src/Pure/theory.ML	Thu Jul 04 14:20:47 2019 +0200
@@ -171,7 +171,9 @@
 
 (* join theory *)
 
-val join_theory = foldl1 Context.join_thys;
+fun join_theory [] = raise List.Empty
+  | join_theory [thy] = thy
+  | join_theory thys = foldl1 Context.join_thys thys |> Sign.restore_naming (hd thys);
 
 
 (* begin/end theory *)